Hello, i am a gold Alistar main , i understand that most of the readers are laughing at the fact that a low elo player is trying to make a "guide" :)
Well i am well aware of my limited capability, but i am a seasoned Alistar player with a decent winrate . I have a lot of fun playing Alistar and i hope this guide can help you guys have fun as well.

PROS :
+His passive heals the AD and himself
+Tons of hard crowd control and displacement
(Q: KNOCK UP ; W: KNOCK BACK ; E:STUN)
+Very tanky with ultimte
(damage reduction)
+Can initiate and break up fights once he have his full combo
+His combo provide surprisingly bursty AP damage even if you build tank items
CONS:
-He get kited easily by ranged champions
-Spell shields, dashes, certain items can negate his combo
-In the early game,Alistar combo makes him mana hungry
-His W and Q have a pretty long cooldown

I like to build both AP and tank items on Alistar, that is why i use the Resolve + Sorcery runes combination:
1. Aftershock is the most suitable keystone for Alistar in due to the fact that his W,Q and E are all hard ccs; When you initiate with Alistar full combo( W + Q + E),in addition to the huge base magic damage from each ability, Aftershock will provide extra damage base on Alistar health and give him some magic resist and armor to survive as well.
2.Demolish is a great option because it will help you take down turrets extremely quickly.
In addition, since demolish scale with health and there are several health items in this build, this rune will be even more useful.
3.Second wind is a basic requirement because Alistar get kited quite a lot during the laning phase.With this rune you can stay in lane for a very long time .
4.Revitalize is the "cherry on top" because it boost the healing from Second wind and your passive.So with all these extra healing, not only will you survive but help the AD stay in lane as well.
5.Nullifying orb function as an emergency spell shield which is great against AP supports like Lulu , Zyra ,...This rune can save you from executes that deal magic damage.
6.Celerity works great with this build due to the fact that not only does this rune grant extra movement speed but it also give you AP/AD scaling with the extra movement that you have.There are two items (not including boots) in this build that give Alistar extra movement speed passively and actively so the amount of extra AP you can get from this rune is considerable as well.

1.Flash is a summoner spell that almost every champion needs due to the fact that it is both a great escaping tool and a magnificent chasing tool.In addition, Alistar can use flash instead of W as a gap-closer.
2.Exhaust can be useful in many different situations such as:
- A form of peel for your ADC when the enemy Jungler or midlanner decide to gank or camp botlane.
-It can also be use as a cc to help the ADC secure a kill
Exhaust is most useful when you use it on the enemy ADC,an assassin or any bursty or attack speed reliant champion.

This build focus heavily on extra Health; Mana; Ability power and movement speed
+The extra Health help Alistar survive through teamfights and peel for his ADC.
+The extra Mana allows you use the W + Q + E more frequently without any mana shortage problems.
+The extra Ability power will grant you the power to burst down squishy enemies if they make the stupid mistake of underestimating Alistar and his "tons of damage".
+The extra Movement speed can help you get in range to use your W + Q combo or to chase down enemies.Plus, this works great with the Celerity rune,from all this combinations you can gain a considerable amount of AP.