I have played Alistair countless games and there are millions of ways to build him. Most can generally agree that AP is important for him, and I agree, as his main purpose is to bring the enemy team to his allies through stun and kick. After playing Alistair a lot, you'll realize that early game, you can rack up a decent amount of kills, you should utilize this, organizing ganks and so forth. However, towards the end of the game, you may notice that you can only get assists. Alistair is the team gank King. Make sure YOUR TEAM is the one to initiate, however, because you are locked down, your team loses invaluable disable.

I prefer AP and CD runes. You'll find yourself running after enemies with both Q and W on CD, the faster these are back up, the better.

You can go defensive, or utility, I like defensive because my items and runes bring down my CD's enough, and I almost never jungle.

http://edge4.mobafire.com/images/item/dorans-ring.gif Gives you the health, AP and mana regen. Even this negligible amount of regen allows your E ability to be spammed, eliminating the need for HP regen. If all goes well, with a few enemy kills/assists before your first b, you should be able to pick up your next item.

http://edge4.mobafire.com/images/item/boots-of-swiftness.gif These are HUGE. This stacked with your passive and the Shurelya's later allow you to catch and disable enemy players. Some people like Sorc or Merc treads, but I think these are vital for playing a good Alistair.

http://edge4.mobafire.com/images/item/sheen.gif or http://edge4.mobafire.com/images/item/philosophers-stone.gif Now your 3rd item is very situational, I like to go sheen or Philosopher's for the shurelya's. Those two items should become your 3rd and 4th items no matter what. If you're doing well, and have several kills, then go Sheen. If you are not getting many kills and not much action in your lane, go Philosopher's for a little longevity.

http://edge4.mobafire.com/images/item/leviathan.gif This is where my build differs from most. Before you complete shurelya's, consider getting this. As a tank support late game, you SHOULD rarely get kills, however, between your pound and passive, assists should be abundant. I find myself around 25-30 assists every game, usually achieving 10 or so kills early on.

Here is where the build becomes incredibly situational. For a more aggressive Alistair finish off with http://edge4.mobafire.com/images/item/archangels-staff.gif and http://edge4.mobafire.com/images/item/lich-bane.gif. These two items will make you do a solid amount of damage with your abilities and even hit hard (not too mention lich bane's effectiveness in tower plowing) If the game is close and you have enough damage on your team already, I suggest http://edge4.mobafire.com/images/item/frozen-heart.gif and http://edge4.mobafire.com/images/item/guardian-angel.gif. You'll still be weak on the MR side, but no AD carry will be able to touch you, and Frozen heart AS slow just makes you that much more valuable for team fghts.

Leveling Pulverize or Headbutt will give you essentially the same effect. Leveling your Roar will allow you to stay in your lane longer, and take some creeps down with your passive. I like to keep Pulverize and Headbutt around the same, as they both do the sameish damage, and are used in conjunction every time.

I've seen a lot of different combinations. http://www.mobafire.com/league-of-legends/edit-build# is obviously good for those scary tower fights. Stun combos work well with heavy artillery at your back. But the most useful ability is your http://www.mobafire.com/league-of-legends/edit-build#. A Flash into enemy team+pulverize will break them up nicely. It can also be used to catch a running enemy. I like http://www.mobafire.com/league-of-legends/edit-build# for the same reasons.

In the end Alistair is incredibly good for breaking up team fights and isolating those pesky carries. One thing to keep in mind is the reverse headbutt pulverize combo. This is useful for chasing among many other of your abilities.

Good luck my friends, and happy hunting.