This is my build for PURE tanking with Alistar.

The rune-page, like with any other champion is flexible, and completely up to the summoner. However, I prefer to get HP Quintessences and Marks, as our build incorporates the heavy tank items such as Warmog's Armor into the late-game. Seals and Glyphs for Armor and MR.

A simple 0/21/9 - where you get mana regeneration in the Utility tree, as well as improved Ghost. Make sure you SKIP Ardor in the Defense tree, as we do not need either Attack Speed, nor Ability Power.

*Again, Masteries and RPs are open to interpretation and should fit your play style.

We shall chooseand.

The reasons:

A) Alistar is one slow son of a *****. Even with our Boots of Swiftness he has a hard time closing in on the team. having Flash, Ghost, and our, will enable us to ALWAYS have means of closing in on the enemy and CC them to death.

Also, all three are quite flexible, and can be used both offensively and defensively.

*Disclaimer: Manage your resources. Do not blow all three cooldowns on one fight, unless it is extremely crucial that you win.

We shall first get our Q skill, or, followed by W, and E.

The reason is, of course, to try to get an easy kill early. Try to lane with someone with a stun, like Sion, to effectively stun-lock and push back enemies at level 2. The result is most often a free kill.

Afterwards, max out your Roar. While some may say that Q and W are better, we shall argue that:

A) Alistar has EXTREMELY low mana early. Ifis leveled early, the mana cost will skyrocket, leaving us OOM constantly.

B) We are here to disable the enemy and tank. Not to do damage.

C) Our mighty roar is quite cheap, and will enable us to keep our lane partner, ourselves, and later the whole team, nice and healed.

This shall be our first item. Notice that we shall have so much HP at lvl 1 that our opponents will be discouraged to engage us. Also, it allows a really good lvl 1 bush gank. Alternatively you could go for your Phil's Stone. I prefer to be intimidating, however.
vs.Both are viable, but I prefer the constant boost of speed.
This is an item made for Alistar! It has everything we need: Health, CDR, Mana and HP regen, and MOST IMPORTANTLY - 4 sec 40% speed active!. Like I said before, nobody will EVER escape you or your team if you combine it with ghost. It's ridiculous.
Once you proceed to our Omen, things get serious. Charging in with Ghost, (or our W) Pulverizing their team, and activating the slow is a ROFLSTOPM combo, which will render their team :O!

*Disclaimer - our Q spell has a 1.5 sec stun. Activate Omen only AFTER the stun wears off! It has a 2 sec effect, and we do not want to waste it when their team is stunned.
We need to get this item solely for farming up our Warmog's! Plus, we need some minion kills too.
Now, let me defend my choice here. Our ultimate provides 75% damage reduction. What's the point in having all this reduction if we have small amount of HP? Alistar needs this item LATE GAME, when people like Ashe, Yi, and the Nuker spellcasters have lots of damage. Otherwise, even with our ULT on, we shall be melted fast.

*Disclaimer: Again, this is LATE GAME. Alistar does fine without it, but it is strongly advised to get one when the game get's close to 50 minutes.

Alternatively, you could opt for a. Get this if an enemy caster like Ryze is doing well. It also gives 8% MS which is awesome on Alistar.

We've felt this growing concern in some of you. "These are expensive items, and Alistar sucks at farming, and doesn't get kills!" Here are several things some of you should consider if you are worried:

A) Alistar is one of those Champions, such as Janna, which don't need a ton of items to do well. This is why Alistar is awesome.

B) Both ourANDcomprise of small inexpensive items! Reverie's ingredients cost 800 and 850 gold. Early on you'll do fine.

C) You would be surprised that you, in fact, will get some kills here and there. Also, a well placed Pulverize can clear many weak minions. Time your last hits, and wait until minions are weak to farm.

D) Stick with your team, get assists, push for Dragon and Turrets.

Hope you enjoy it!