After playing quite a bit with Alistar and trying out different builds I found this to be my favorite way to play him.

This is a simple guide and my first so it's not that fancy, but hopefully some people will find it useful

Alistar is a has a long cooldown on his abilities and his Q and W eats up a bit of mana. That is why I like to have mana regn. marks and some CD glyphs.
His one swiftness quint is because he is very slow until you buy boots of mobility.

Here I focus alot of points in the tank department because lets face it he is a tank.
And the swiftness rune I used allows me to use mastery points on mana regn.

I start with the heal so I can fill me support duties. then I rank up W first then Q, but that can be up to yourself.
Then when his ult is ready you can tower dive and feed your AD Carry.

A combination I like to use is Q an enemy then W him back so your team or/and AD Carry can get extra hits on him and pick up the kill.

Ofc you cant hold all the items I suggest, but it's to make some options for the different games you may encounter.
So if you meet a heavy AD team take the amor items and if you meet a heavy AP team take the magic resist items.
But the best combination would be Force of Nature and Frozen Heart, but those are expensive so do it if you have the money don't if you haven't