---UPDATE--- Alistar's AP ratios were nerfed yet again, don't even try AP Alistar, just follow the 2nd and 3rd builds, and try to ignore the first. Welcome to my guide to AP Alistar. AP Alistar, not that common, but certainly fun, and dangerous.Alistar should be played as a roamer(Philosopher Stone/Kage's Pick/Heart of Gold)OR, more commonly support(Aegis of the Legion being your core item) unless you get into a situation where AP Alistar is viable, which is rare in high level play, as the make-up of teams is usually better than that. Alistar is easy to pick up, cheap in IP (1350), does mass damage (headbutt can do up to 800 or more with this guide) and is still a tank (if you're lucky like me, and are always paired against 2 carries to lane against), due to his ult. Though keep in mind, you can still solo a mage (I.E. Malzahar, Ryze) but if there are 2, you can NOT just run up, kill one, and run away as I usually do with AD carries and such. Your R reduces physical and magic damage!
I usually play 3v3, (personal preference), but this certainly works in 5v5, as well.
*UPDATE* Headbutt no longer uses 100% of your AP.
*NOTE* AListar's R can break CC's, so use it if you're being lasered by Malzahar.

Alistar, in this build, uses ALOT of AP, thus the magic penetration runes, and this is an AP build, so AP glyphs are nice, the MR and armor is to make up for what you are lacking because you're building AP, and the 2 move speed quints because Alistar has a tendency to be slow.

The reason I use ghost is because Alistar is extremely slow, and on the rare occasion you are being murdered, it helps a lot.

Flash is probably one of the best abilities I'd say you could choose for Alistar. Great for tower diving, just flash behind them, knock them up with Pulverize, then headbutt them towards who ever you are laning with, and out of the tower they go. Also good for escaping, if they are already right next to you so you aren't going any closer to them.

The 3 in offense are for AP, but the choice of dodge in defense might seem odd, as I don't have any dodge runes or items in this build, but it helps more than you would think. The health and mana regen shouldn't be too wierd looking, but the mana increase is there simply because I have nothing else to choose that I think is useful, or that I see.

Get headbutt first. In 5s, if they get the jump on you at level 1 (I.E. you walk into a bush and the 2 people you are laning against are there) you can headbutt one away from you and run, and the other usually won't chase you on their own. This also does more damage than your Q, although it is a single target ability. I mainly level Q next, (Pulverize), because you want to dish out some damage early on, and you should be fine on health so far, with that single health potion you bought. E directly after that, because chances are, you're being focused and used up your health potion. Spam this ability! Return to buy and get more mana potions regularly so you don't run out of mana so fast.

Cons:
- Slow
- Focused often
- Easily ganked
Pros:
- Insane damage
- Insane Survivability
- Pentakills are highly likely (have gotten 1 pentakill in the few 5v5s I've played, several triple kills.)
- Off-tank material
- Tough
- Destroys carries
- Easy to get away (unless they all have ghost and exhaust)

Ionian Boots Of Lucidity - You need cooldown reduction more than tenacity or spell penetration.
Sheen - Great damage with this, pop your R, headbutt then into a wall, auto attack them, pulverize them, then another auto attack. That is about 920 damage right there (Before armor and magic resist, around level 8)
Rabadon's Deathcap - This will quadruple your AP if all you have was sheen and r2 boots, no need to explain further.
Lichbane - Move speed, magic resist, ability power, mana, and a super sheen passive.
Rylai's Crystal Scepter - Health, AP, and a slow when ever you cast an ability on someone.
Zhonya's Hourglass - Armor, AP, and the stasis is nice.
Morello's Evil Tome - Mana regen, AP, and cooldown reduction. This thing is perfect AND cheap.

If you aren't being fed, consider getting Morello's in the place of Rabadon's, and wait to get Rabadon's until the end. Not only does the cooldown reduction help a ton, but its about 1350 gold cheaper.

Another advantage to AP Alistar is that his passive stacks with AP, making farming easier (and accidental kill-stealing). Be sure to run over as many minions as you can after you cast anything.
At later levels (start at around 11, if you're fed enough), when you have enough mana, use Pulverize to farm. Go up to the squishy caster-type minions who fling blue/purple lightning balls and cast it, then run them over at most twice, instant 100 gold. If you don't have a kill by level 6, build a Moorello's Tome (sorry if you don't like puns :]) first. The CDR and mana regen help you spam your abilities and let you stay in your lane to make up for the gold you didn't get from kills and even out level your enemies.

Teamwork goes a long way as ANY tank gone AP. Headbutting a squishy Karthus out of his tower and into your Olaf... TEAMWORK!
The old Lee Sin, Alistar, and Xin Zhao combo. (can you see where this is going?)
Start off by you and Xin rushing at someone while Lee stays behind. Once you are close enough, pulverize him, let Xin Zhao activate his knock up, and while he is doing his triple auto attack, run behind whoever's day you are ruining, and headbutt them towards Lee Sin, at which point, they should nearly be dead, so to finish them off, Lee Sin uses Dragon's Rage and roundhouse kicks them BACK at you and Xin Xhao on the off chance that they are STILL ALIVE.
There is absolutely no reason they should be able to make a comeback and kill one of you.

This build is straight up AP Alistar, nothing else. I don't think I've seen one of those yet (that has been updated so that it doesn't include nonexistent items anymore, I.E. Zhonya's Ring), and the potential to get multi kills is amazing. Alistar has 2 disables, a knockback, and a knockup, and during the duration of those disables, they can't attack you, unlike a snare, so not only are you doing a ton of damage, but they aren't. Also, headbutt stacks with your AP 100% I.E., if you have 500 AP, you're going to do 800 Headbutt damage(nerfed, now only 80%) (before magic resistance), I have yet to see any other champion who has an ability that does that. Another thing is that once you activate your R, you also gain 60/75/90 attack damage, which means that if they are stacking MR, you can just disable them and auto attack with your Sheen or Lichbane and still do massive damage.
Once you get all your items, you have potential to do 800(Headbutt)+600/690(Lichbane auto attack, more with Unbreakable Cuffs (Unbreakable Will))+750(Pulverize)+600/690(Another Lichbane auto attack)=2750/2930 damage, all in about 3-4 seconds. Keep in mind that this is all before resistances, so don't get cocky thinking you can solo Jax and what-not. *UPDATE* Headbutt NO LONGER uses 100% of your AP, everything above might be incorrect.

Your R is amazing, and the last rank has a super low cooldown, and you can usually use it every team fight. If you're matched against a Tryndamere and your team isn't abnormally bad, you're going to win. When Tryndamere uses his ult, use yours, he won't be able to kill you. But, if he doesn't have to use his ult because he's beating you, you aren't spamming your abilities enough, and you probably should just play Yi (joking, please don't.)

I feel that Alistar has some jungling potential, with his heal and passive. If someone could test it out and tell me how it goes, I might consider making a build for a jungle Alistar.

I have come out of 3v1's victorious, and still with around 40-50% health left with this build. Alistar has quickly become my favorite champion, and hopefully this works as well with you as it does for me. Constructive critisism is welcome, please do so if you think something is off/could be improved, as that would probably help me as well, not just other people reading this guide.

If you don't know what roaming is, then I suggest you don't use that build, although running around with 70 HP per 5 and 40 mana per 5 is fun. If you really don't know and really want to know, roaming is running around between lanes during the laning phase, with no definite assigned lane. Roaming is meant to be used in 5's only, it would be kinda bad in 3's. Philosopher's stones are perfect for roaming because you need the regen because you are not supposed to recall very often at all, because experience is very sparse if you are roaming, and the gold per 10 is great because the opportunity to last hit minions is sparse. Roaming basically gives another solo lane to your team, meaning that if you have a jungles as well, that is 3 solo lanes, and you're there for those 3 when they need it. Support Alistar is meant to be bot lane in 5's if you are building support. You are basically just there for your CC and heals, so utilize those as much as possible.

Mana-Mana/MR-Magic Resistance/AD-Attack Damage/AP-Ability Power/HP and Mana per 5-per 5= per 5 seconds/MP-Magic Penetration