This is the first guide ive written so if I make any noob mistakes please tell me. When I play Alistar I try to increase His team fighting ability by purchasing some of the best team support items in the game, and purchasing these item doesnt prohibit your own performance because these items also fit Alistar's needs perfectly. I've seen some good guides for Alistar, but never anything that truly satisfied my playstyle. So I decided to make a guide how to play Alistar to his full potential.

Pros:

One of the best team fighters in the game.

Basicaly can't die while ultimate is active.

3 AMAZING support moves (one of them being a AoE heal)

An awesome minion pusher.

Destroys turrets in seconds.

Extreamly underrated.

Super fun to play!

Cons:

Kind of hard to farm.

Not the strongest guy ouy there.

Mana can be a problem early/mid game.

Runes are pretty self explanitory. Damage reds, mana regen yellows, and mana regen blues.
There are a few other options for runes but IMO these are the best for Alistar's needs.

I go 9/0/21 with my Alistar to get cooldowns on my ghost and flash. As well as getting more gold over time and some much needed mana regen. I've seen Alistar get 21 in defense, but I don't really find it necessary because he gets enough defense and hp on his own, and I think the mana regen compensates for the extra health and defense.

Flash and ghost are probably the best choices for Alistar as it aids him in initiating team fights and getting away if needed. The only other spell that would be effective on him would be teleport, so he could back door turrets with his ult. Either way is fine, but I think ghost is more useful.

As far as your skill sequence goes. It doesn't really matter too much as long as you keep pulverize low until mid/late game. The reason for this that pulverize is used for the stun, not the damage, so its best to keep it at one to help conserve mana for your heal when needed. Try to keep heal and head-but pretty even so you don't have puny healing power.

There are TONS of ways to build Alistar, but I feel that he is made for disruption in team fights and I built him this way so he could just that to his greatest potential.
Try and start off with a mana manipulator unless the person in your lane doesn't use mana. If thats the case go for a sapphire crystal and a hp and mp pot. After getting some cash buy a glacial shroud or sheen. From there you should be pushing pretty well with your lane partner. On your next recall, get some boots and get the glacial shroud or sheen. (the one you haven't purchased) You should be pretty tanky with the glacial shroud so try to get your lane partner to start doing some heavy harassing while try to knock them around. On your next trip, try to get some merc treads and if possible finish your frozen heart.
After this point there is no right way to build your Alistar. Its all up to what you think is necessary to get depending on what kind of chars the enemy team has how they built their characters.

If they have a decent amount of magic power, get a banshee's veil to turn their fireballs into sparks.
If mages aren't a problem, get a soul shroud to give yourself some extra hp, and you and your team a huge cooldown reduction.
If your team is pretty squishy and your having trouble protecting them. Grab a legion to give them a little extra boost.

Alistar has a little trouble getting last hits without taking too much harass. With this being said, its usually best to have a team mate with good harass capabilities with you.
On the very first minion wave you should pulverize the enemy creeps to get a huge head start on pushing your way to their turret. You should pulverize every minion wave with the cannon creep. Why? Because stunning the cannon creep and his friends help your minions take less damage. Its not that much of a difference, but every little bit counts. You should also pulverize when you see the opportunity to last hit 2 or 3 minions to get a nice gold boost, don't be afraid to just heal your minions! If your pushing the turret hard healing your minions means the difference 10 and 15 seconds of time to push that turret! Not only that, but it activates your sheen to do some extra damage. Popping your ulti to push your turret isn't a bad thing either. Your ulti should be at around a 50 second cooldown with your cooldown reduction anyway. Your ulti adds about 100 damage to turrets mid/late game so do it!

And here we are to Alistar's best attribute. TEAM FIGHTING! To tell you the truth team fighting with Alistar is fairly simple. There some things you need to know while in a team fight.

Flash makes a HUGE difference on how effectively you can initiate! Try to save it for teamfighting if at all possible!

Spam heal! The best way to do this is wait for their creeps to get where your team is so you can spam your heal incredibly fast when they start dying off during the fight.

Use your ulti only when they start to target you. Wasting anyone's ulti is a bad thing and it's not any better for Alistar.

Head-but the champion with the most crowd control! This is probably the best way to use your head-but in a team fight. If their team seems pretty even with cc. Try and headbut their carry or ranged dps (Ashe, Kog-maw) into your team.

Please be nice as this is my first guide! I hope I can prove to all of you that Alistar is an AMAZING champ when used in the right hands.