He is great early game, spamming Roar, keeping minions and allies alive, pushing when he can, soaking up damage and pushing enemies away. Late game, he becomes less effective at things besides soaking up damage and occasionally punching a tower.

Well, bother that nonsense. We are going to tear some shit down.


So, what, no tanking?

Screw that. People look at Alistar and see a tank who happens to be rather effective at killing towers. Those people build Alistar as a tank, and then he says he sits around not dying, then occasionally shows off some sweet particle effects and doesn't die harder. He occasionally reaches a tower, whereupon he hits it for slightly more than the carry-mode Ashe--earning a rating of a decisive "meh". 

I see Alistar as a living wrecking ball, who happens to be alarmingly durable. A bulldozer, who, coincidentally, is in fact a bull. When you play him as such, and play aggressively, the result is balls-to-the-wall awesome action. 

Basically, the point is, herein I will propose an aggressive, offense-minded Alistar build, emphasizing his offensive abilities, with his defensive abilities playing a secondary role; in addition, I submit an equally aggressive philosophy and strategy to supplant his normally stalwart mindset. Let's see a quick breakdown supporting my point.


Eat Bullets!

• Alistar is already head and shoulders ahead of nearly every, if not every character in terms of HP, Armor, and Magic Resist--it doesn't take much to make him an outstanding tank. Going overboard on tanking gear is just that--going overboard.
• Alistar can hit harder than pretty much anyone, he just has bad attack speed. Throw in the massive damage bonus from Unbreakable Will, and the even more massive damage bonus from his passive, and you are looking at an absolute one-man wrecking crew.
• Alistar's abilities are phenomenal gank machines if you can capitalize on it--if you go offensive, you CAN capitalize on it.
• If you focus on his offensive abilities over his heal, he can harass like mad. Keep in mind that Alistar's version of "harass" is "walk straight past all the minions and cave their skull in with fists so large they could be dedicated as national monuments, then win because you can hurt damn near anything way worse that it can hurt you".
• Most "smart" players will only get involved in a fight if their mental math shows them winning the damage race. Two punches in, they will figure out they aren't going to do that, and will run. This gives you a huge intimidation factor early on.
• On top of that, most people mentally associate "Pulverize" with "dying miserably to a gang-bang". You often won't have to do more than smack somebody with Pulverize once to get them to run away, because they assume it's the set-up for a rapefest. Granted, it usually is, but...
• Between defense masteries and defensive runes, you can replace one mid-tier defense item anyway.

Basically, Alistar is durable enough on his own that your masteries and runes will give you enough durability that you ought to be looking at items that make him more dangerous; a mobile weapons platform--just add the weapons.

(A side note: From here on out, I'm going to refer to the standard combination of Pulverize, walking in front of them, and Headbutting them back towards your allies/minions/tower "the one-two punch". It's his deadliest trick--learn it well.)


Runes

After much experimentation, I've found this combination complements Alistar's natural abilities excellently, and allows him to be more than just a sponge.

Marks: Desolation (Armor Pen.). This is the cheapest, easiest way to maximizing your damage versus Champions and Towers alike. 9 Marks get you 20 Armor Pen.
Seals: Shielding (Scaling Mag. Resist) This will alleviate the spike damage that your Roar won't wash away so easily--generally, physical damage won't be an issue unless you're fighting the deadliest of physical fighters (Jax, Yi, Ashe, etc.). This will also reduce the amount of money you have to spend on durability items.
Glyphs: Clarity (Scaling MP Regen.). This is specifically for use with the Chalice of Harmony, your go-to item for long-term Mana-management. After about level 4, you will almost certainly have enough mana to use the "one-two" when you need it.
Quints: Avarice (g/10s). This may seem weird, but the fact is that Alistar is a poor farmer, which means buying specialized equipment is difficult. Avarice runes give a significant boost to your income that can't be reduced by being dead, helps even when you're traveling between lanes, retreating, etc.

If you want to experiement, you could try Swiftness in Quints, since I won't be recommending move3 boots. You could also try going for more of any of the above, to suit your personal tastes.

Masteries

0/21/9. Make sure to max out both the Armor and Magic Resist masteries, and the mastery for Cleanse (seriously--just keep reading); the rest you can figure out. The 9 in utility is for Greed, for the same reasons as Avarice above. If you decided to take Swiftness Quints instead, consider going deeper into Utility for the 3% movement speed--there's no kill like overkill.

You could make a good case for offense tree, and you could be right--I haven't tested it yet. More news here after testing.


Summoner Spells

• Flash is a must. Besides being a "get-out-of-jail-free" card, it's also the perfect setup for the Pulverize/Headbutt "one-two punch", which will almost always result in a kill or a forced retreat.
• Cleanse is invaluable, for many reasons: between the necessary Flash and the fact that Unbreakable Will is itself a Cleanse, having a third "get-out-of-jail-free" card, especially one that cools down in a piddlesome 90 seconds when you have the mastery, basically means you can get out of anything, nearly anytime. This is key--it allows you to be indestructible without having to spend all your money and effort on expensive tanking gear. The enemy will think you're far more durable than you really are; they don't realize that they're not hurting you not because you can withstand their attacks, but because you're not allowing them to attack you--somewhat opposite of most tanks.
• If not Cleanse, Heal can be great for early game, since it'll make the enemy even further underestimate your durability, which means they're that much more likely to get themselves killed.
• Promote even further emphasizes your role as the tower killer, but its cooldown is terribly long, and its effectiveness can depend largely on who you're up against.
• Teleport can help with Alistar's slowness, but honestly, any of the above will be more influential on the game outcome.

Items

1) Meki Pendant, 1 Health Potion, 1 Mana Potion

Regrowth pendant would normally be the first, but I've found you can actually subsist pretty well on just Roar--don't bother. Start with Meki instead--you'll be needing the mana.

2) Meki Pendant-->Chalice of Harmony

Alistar's classic weakness, aside from low attack speed and intense body odor, is his god-awful mana pool, trying to support two of the best ganking moves and the most spammable ability ever. However, both of his offensive abilities have rather long cooldowns, meaning that while you'll need a heap of mana at once, you'll only need it in one burst, at a maximum of every 15 seconds--and honestly, the opportunity for a gank just won't present itself frequently enough for that to happen. Chalice will ensure you always have just enough mana for your attacks, while also giving you a hefty Magic Resist bonus--you should be set on Resist for a while if you get this early enough.

Alternatively, you could start with Sapphire and upgrade to Tear of the Goddess, which will work well with his Roar-spamming, but my testing has generally seen this as being a little less useful--it's a bit of overkill early on, but you will eventually run out and be helpless--Chalice has no such weakness. Although Tear has the bonus of building into Archangel's Staff, Alistar just doesn't need an Archangel's Staff, and the Resist on Chalice is a bit more valuable. If you really prefer this item, I would recommend you ditch the Clarity runes for something else, like Celerity--lower cooldowns can take advantage of your unnecessarily over-sized mana pool.

3) Boots

The bull's gotta move to stay aggressive. This is the one spot where you'll have to choose your upgrade carefully:

• If the enemy is big on physical DPS--and I mean big, like Jax or Ashe or something--go Tabi. If you took the masteries for improved dodge and move speed bonus when you do, this is truly fantastic. But honestly, you'll only need this if the enemy has the absolute strongest physical champions, and even then, only the ranged ones--the melee you can either beat or one-two away.
• If the enemy is Team Stun Spam, or team ultra-nuke, go for Mercury Treads. This is even more awesome with the above-mentioned Flash and Cleanse--you'll basically be impossible to pin down!
• If the enemy just doesn't seem to be exemplary in any one stat, or you're out-leveling them somewhat, go for Berserker's Greaves. Even this little bit of attack speed will increase your damage significantly, especially against towers--every second counts when smashing towers, particularly when using your ultimate. In addition, your next attack speed item won't come until pretty late, so this will have to do for now.
• If you find yourself moving very rapidly between lanes, or burning out even your beefed-up mana supply quickly, go for Boots of Mobility to help keep up--also a good surrogate for Teleport. This can be invaluable for setting up ganks, as headbutting an unwitting enemy into a tower or harmlessly to the side ("way to ruin the gank, Al!") can come down to a fraction of a second of reaction time.
• Generally, I don't recommend move3 boots, because although they can help ganks (as Mobility above), they're best for chasing, which is generally not something Alistar should ever do--you're almost certainly not going to be outrunning anybody. When a kill is going to happen with Alistar, it's not going to be because he was 20 points faster than an un-slowed enemy--either you catch them completely unprepared and they die, or they run and you kill their towers. Force the enemy to respond to you, and you won't have to worry about chasing speed. The extra stats on the other boots are a little more valuable than the possibility of risking yourself and getting that one more kill. You push towers better than you kill, and fortunately, pushing towers down makes you win more than getting kills.

4/5) (Choose One: )

• Atma's Impaler: Haven't bought this beauty since the nerf, eh? Well, get used to it, because it's the perfect package for Alistar.
  • It'll easily give him a good 40 damage without needing an HP item--you get so much HP naturally, it'll be more than enough, and you get so much damage naturally, 40 should be sufficient.
  • 18% Crits is no joke, and your punches hurt a LOT already, especially with all that armor penetration. Nice for the two good slaps you'll get in after a good Pulverize.
  • 50 Armor = WAYYYY less damage from physical sources (towers, most importantly), and you didn't even have to buy a defense-only item to get it!
• Sheen: Also a must-have for Alistar--every part of it is invaluable to him.
  • The extra mana is just enough to ensure you definitely don't have Mana problems ever again.
  • The AP adds 1-to-1 on your one-two and 30% on your heal--that can make a bigger difference than you might think.
  • The unique passive works wonders with your other abilities--it turns the "one-two" into the "one-two-THREE", and Roar is the perfect enabler. Whenever you attack a tower, just spam Roar as hard as you can, and your damage will skyrocket.

The only question with these two is, you really need both--so what order to get them in? Basically, if you're dominating whenever you get into fights, get Sheen first to REALLY capitalize on your bullying power. If the enemy is overpowering you a bit, go for Atma's first, starting with the Chain Vest if they're physical (Armor helps more than HP unless you already have a ton of it), Giant's Belt if they're magic-based.

I need to do some testing, but I'm looking at what happens if Sheen is your first item (starting from Sapphire), and you get Chalice after that. It's a bit longer to wait for boots, but again, the kills will happen when they happen, and it won't just be because of your speed.

6) Agility Potion

If you got the change to throw around, get this, because odds are, if you've been aggressive enough on the towers, the game is probably going to be over before you get another major item. This little beauty will push your crit rate to around 60%, and even further increase your attack speed. Very valuable if you didn't get Berserker's Greaves.

6) (Choose One: )

• Need to hit harder? Last Whisper is the cheapest way--far cheaper than any decent damage item (damage that Alistar honestly doesn't need much of), but still greatly boosts damage in an indirect way. The hefty attack speed bonus even further maximizes your killing power on people and towers. By now, you should be attacking so fast, you should be able to take towers from full to critical (if not dead) in just one Unbreakable Will.
• Still not standing up to the enemy team? Banshee's Veil--hefty magic resist, health, and mana (not that you need it), and a fourth get-out-of-jail-free card. Now that's a fine deal.
• Need a little of both? Frozen Mallet--absurd HP, high damage bonus due to the item's inherent and the Atma's bonus, and a near-guarantee that the "one-two punch" will be followed by punches three, four, five, and six.

Skills and Strategy

Start with Pulverize. I normally would recommend Roar first, but having a nasty weapon like this to pull out will save you from early-game hijinks more than you could believe--nobody will stand and fight after getting launched, stunned, then punched for 70. Besides, you bought a health potion for this, right?

Try to lane with another disabler, like Morgana, Ryze, Taric, etc. Singe or Blitzcrank are ideal partners; Fling and Rocket Grab, respectively, are absolutely lethal when combined with your "one-two"--let them get the enemy to you, headbutt them back, then stun them where they land--if they were closer than halfway down the lane, that's probably in turret range, and probably a kill. This is much easier if you're in the middle lane, as there's less distance to be traveled to get in Tower range. 

This is ideal situation, of course, but if you have this excellent pairing, you can afford to be even more aggressive--the rewards are far greater, and even if you fail, the enemy will now be terrified of you, which is almost better. Getting to the enemy player can be much more valuable than just killing their Champion.

As a side note, when to use Cleanse: use it when you get stunned/rooted/significantly slowed, immediately, but only if they can actually throw damage at you; sometimes, disablers will throw a "token stun" at you, even when they can't actually take advantage of it, just to remind you that they can and keep you scared. That said, always break out of disables that have damage over time effects, like Morgana's Binding, Ryze's Rune Prison, or Teemo's Traps. 

Cleanse cools down so incredibly fast that you might as well fire it very frequently--it'll keep your health far higher, and the enemy far more frustrated. They should begin to understand that you will not dieunless they send everyone and everything they got. Alistar cannot be easily harassed, they will have to commit if they want to seriously hurt you. This is "Lesson 1".

Level 2, take Roar, fire when necessary, but you honestly should keep your HP around half--just high enough that nothing can spike you down, but low enough that the enemy might get dumb and try. When they mess with the bull, give them the... well, you know.

Level 3, Headbutt. Now you can inflict some serious damage. From here on out, never be afraid to take the opportunity to give them the "one-two", but don't run them down for it, either--just decide to punch a minion that's a little closer, then the next closest one, then the next... if they don't drop all kinds of hurt on you, and they don't get the hell away from you, "one-two" then go attack the other guy, if there is one--the guy you just launched will definitely be too focused on running, while his partner will be trying to "punish" you. If you start hitting him first, he'll have to fall back, too. Walk away, roar, and carry on. Eventually, they will learn that you will fight them at the drop of a hat, and you will probably win. This is "Lesson 2".

Level 4, take Roar again--you can now heal hard enough that you probably won't need to put any more points in here for a while. Try to time the heals right before minions die so you can minimize cooldown.

Level 5, take Headbutt--it scales damage a little better, and it can be used to kill a foe you're chasing in the soon-to-come brawl.

Level 6, take Unbreakable Will, and ascend to building-bashing godhood. For the rest of the game,whenever Will is cooled down, you need to make reaching a tower with a minion wave your number one priority. The second you get within reach of a tower that's notshooting you, immediately pop Will and start pummeling the tower. Yes, it's nice to have it to get you out of a stun or something, but until Heimerdinger rears his ugly head, nothing can repair towers, and you can hurt them harder, faster, and more than anyone else--and, conveniently, nothing can hurt you enough to stop you from pummeling it until you feel like stopping. So, "Lesson 3" is, the enemy team can't afford to let you live.

From here on, focus on pumping up Pulverize and Headbutt (preference to Headbutt, for the moderately-better damage during chases), Will when you can, and just keep hammering towers. As long as the enemy team is scrambling to "stop" you, you have the advantage, particularly with all your escape mechanisms. Generally, this means you won't be moving with your entire team unless

1. They're pushing faster than you, in which case you need to even further accelerate the destruction (Sun Tzu say, rapidity is the essence of war )
2. They're in a team fight they won't obviously win, because if they lose, the enemy team will either all come and stop you, or will counter-push a lane, which is far worse.

Having one or two guys, probably the disabler you laned with as your wingman, is probably best--this way, the two of you can either kill or force any one- or two-man team that comes to stop you to retreat--they'll have to send everybody to stop you, which clears the way for the rest of your team.

Above all, don't die unless you take a building or a person or two with you, but if you have to choose between escaping and finishing a tower, unless the tower is rather low, you might as well finish the job--you don't know when the next time you'll have this chance will be.
Conclusion

This is still under testing, of course, but if anyone wants to test playing an aggressive Alistar and let me know how it goes, it'll help me touch up things a bit. All in all, hope you enjoyed the (massive) read, and enjoy the bulldozing!