I like people knowing little about me, so introduction is MIA.

Pros:
+Has a knockup that gives him awesome positioning
+Gapcloser/opener
+Almost Spammable AoE Heal
+Itemless Tenacity
+Can't be milked

Cons:
-Extremely Mana dependable
-Unviable early game
-Zero Damage

Q: Why these runes?
A: The build you are about to see doesn't give much tankiness to Ali. It makes him just squishy, so runes are there to do something about it.

Flash Just too OP. Combo included: Flash+ Pulverize+ Headbutt=kill secured
Clairvoyance It might happen that you run out of wards, spammable, gank-prevent=90% if you know where to spam.
Clarity As said, Alistar is extremely Mana-dependable. Also, you are no Soraka to spam E every once in a little while on your AD carry.
Exhaust You are a tank, but you don't have slows. This SS grants your wish of slowing down a chasing/fleeing enemy.
Ghost is somewhat good on Alistar. Gives you speed, enemies see you coming, you scare the **** out of them before you Pulverize+ Headbutt them.
Barrier Just no. You have Locket of the Iron Solari, which shields you AND your whole team. Supports aren't supposed to be selfish.
Cleanse Two words. Unbreakable Will
Ignite Do you need kills? Seriously?
Smite Wait, is this Junglistar? Oh well, then nope.
Teleport Really?
Heal. Pfff. Triumphant Roar should do the trick with less CD

Trample Deals magic damage around Alistar and makes him ignore unit collision when he pops an ability.
Unnecessary while supporting. It just makes you accidentally steal farm from your AD carry and makes him rage very often.

Pulverize Deals magic damage and knocks enemies around Alistar, stunning for 1.5 seconds.
Bread and butter CC in teamfights. It can make the whole enemy team ragequit if it's combined with

Headbutt Jumps to an enemy, deals magic damage and knocks him away.
The initiation is on you in teamfights. Just Flash+ Pulverize+ Headbutt the enemy AD/AP carry and GeeGee, you win the teamfight.

Triumphant Roar Heals Alistar and all allies nearby. Cooldown reduced each time an enemy dies.
Make sure you AD carry farms a lot and fast, and you have infinite sustain, if combined with Clarity

Unbreakable Will Gives Alistar AD and reduced magic damage.
The AD component is useless when support, but the reduced magic damage makes AP carries rage. A lot. Also serves as a lower CD Cleanse

No need to explain.

Game Start
Total Biscuit of Rejuvenation + Explorer's Ward + Faerie Charm + Vision Ward + sight ward + Health Potion + Health Potion
Just a classic support start, no biggie.

First ten minutes of the game
Faerie Charm -> philosopher's stone + Boots + Sightstone
philosopher's stone will be upgraded into Eleisa's Miracle. After that, get another one.
Boots=early game speed that does a lot of dodging.
Sightstone. Wards. Wards everywhere.

Core
philosopher's stone -> Eleisa's Miracle + philosopher's stone -> Shurelya's Battlesong + Sightstone -> Ruby Sightstone + Boots -> Ionian Boots of Lucidity w/ Enchantment: Captain
Why Eleisa's Miracle? Well, it gives you 30% CDR for your CV, and HP and MP regen, without taking a slot. Try to rush it pre-10.
Shurelya's Battlesong. Gives you AND your team a quick steroid of speed every minute.
Ruby Sightstone. Again, you are the support, you ward. This item saves lives. Sell it late game for Banner of Command
Ionian Boots of Lucidity+ Enchantment: Captain. The CDR is exceeded in total by 5%, but 40% is better than 30%. Also, Enchantment: Captain gives your team a perma-boost of speed. Also, it's very underrated, but just too OP while chasing or fleeing. Give it a try. If it doesn't work, Enchantment: Homeguard should do the trick.

Final build
1) Mikael's Blessing. Sustain level over 9000. Also gives Cleanse to other team members. This item gives Honor:Teamwork to you every game.
2) Locket of the Iron Solari. Team gets shielded, team doesn't get pwnd that easily, team wins teamfight. Dead simple.
3) Banner of Command. Take it earlier for sieging and gold, because it makes enemies rage.
#tons of damage
4) Runic Bulwark. Aura of effect magic resist and armor steroid. All in all, your team just tanks up when you are nearby.

Good allies
Vayne. Easy. Headbutt+ Condemn
Tristana. Same. Headbutt+ Buster Shot
Ezreal. A good carry with a great disruptor gives tons of damage.
Graves. Same as above.
Katarina. Your role here is just Flash or Headbutt into Pulverize. She does all the damage. # Death Lotus
Kennen. Very similar. # Slicing Maelstrom
Maokai. Engage with Flash into Pulverize, then Headbutt the AD carry. He won't move until he respawns.
Galio. You Pulverize, he has enough time to do****. Best description ever. Do****.

Easy enemies to counter
Blitzcrank. If you mess with the bull, your team dies.
Make sure he Rocket Grabs you, after that it's easy: Pulverize, then Headbutt the carry.
Brand. His combo is so easy to disrupt. Displacing him should win the teamfight.
Caitlyn. Headbutt > Ace in the Hole.
Fiddlesticks. He charges Crowstorm, you easily Flash Pulverize him. If not fed, you also Headbutt him into your team.
Galio. Step 1: Make sure you're out of his ult.
Step 2: Headbutt him, defying his damage and thus making him useless for the rest of the teamfight.
Katarina. One word. Headbutt.
Kennen. Same
Malzahar. Nether Grasp is a channeling spell, so it's same as above.
Morgana. Neutralize her Soul Shackles ftw.
Leona. We thinks it speaks for itself. #gollum
Nunu & Willump. Same as Galio, but easier.
Miss Fortune. As all channeling ults are countered by Alistar, so is Bullet Time
Ezreal. He Mystic Shots, you are in his way, denying any possibru damage.
Thresh. Remember Blitzcrank?

Alistar is bad to pick against:
Ahri. Easily escapes Flash Pulverize
Teemo. Well, Move Quick should get him out of the range of Headbutt.
Fizz. You try Pulverize, he dodges. You try Headbutt, he dodges again. Fizz OP.
Elise. Neurotoxin/ Venomous Bite deals %HP damage. Elise OP against tanks. Cow pls.
Olaf. # Reckless Swing. #true damage. # Olaf OP
Kha'Zix. Since the new patch, he deals % damage from level 1, without evolving. Good luck.
Janna. Truest counter support for Alistar.
Vayne. With Silver Bolts, a tank has been slain.
Lulu. Headbutt = Free Whimsy.
Soraka. Bananas so annoying.
Kayle. Although she is just weak on bot, she is too op. # Intervention.

#OP.