Why not an AP Alistar?
Why be an AP alistar when you have other casters or melee heroes to take that role? Every successful team needs a tank to take the damage from other players. Alistar is a great example and is in my opinion the best tank there is because of his self heal and his base health. taking advantage of this self heal and both his stuns could be the turning point of any team fight.

When going with a tank alistar, you want to max out your heal spell first so you can take all the damage you can. When you cannot be leveling your healing spell, upgrade your pulverize (the spell where you hit the ground) so you can get that edge on the opponent by stunning them for those few seconds.

When you are done leveling your heal spell, start focussing on your pulverize and head butt, while also maxing your ultimate (unbreakable will).

The main role of a tank is of course to take the damage, but also to help your teammates take out the enemy as fast as they can. You should use you pulverize and head butt to your advantage by hitting them in the air first, then run under them, and finally push them backwards to make them get away in the most time possible. Keep your heals on cooldown always by spamming them to keep yourself and your teammates alive.