Alistar is a very strong top laner who is very under rated.

When built correctly, he is highly damaging and extremely disruptive. He is capable of soloing carries and zoning them out of team fights.

Once he builds Iceborn Guantlets he becomes an excellent pusher and farmer. Who, with teleport can split push well

In lane, he has extremely high sustain and can peel melee off him all day long. He accepts ganks really well and he resists ganks really well.

His harass is on the low side until he completes his Sheen but he's fully capable of winning his lane using the help of his jungler and turret.

Once he does complete his Sheen coupled with the philo stone for mana regen, he can bully melee out of lane.

Because he wants to build armor first and because he does best against melee, his biggest counters tend to AP champions, especially manaless ones and Cho'Gath. Even against his worst counters he can survive and transition well into mid game and also have amazing teleport ganks.

The truth is, Alistar can do everything that Shen can do, except better.

The main idea behind Ali is not to get yourself kills, but to enable kills for your teammates.

As such you run utility summoner skills and masteries to help you function as a bruiser and tank without an extreme amount of gold.

You focus on CDR as your main stat. The stat will give you tons of utility and damage due to Sheen and Liandry's Torment.

Once you have achieved full CDR, you build full tank and put yourself in harms way for your team. This will force their carries to focus an Alistar. A 4k hp Alistar with tons of disruption, self healing, and an ult that makes him take no damage. If they don't focus you, you beat the tar out of them, constantly disrupt them, and ruin their positioning getting them killed.

Teleport:

Teleport is very important for top lane Ali. Just as important as Shen's ult is for his kit.

warding/vision is key here. The wards will allow you to see enemy ganks coming and hence counter them. The wards also give you something to teleport to.

Make sure your bottom lane has wards or teleport isn't as good hence top ali isn't as good.

Flash: Flash is soooo broken on Alistar. Flash allows you to instantly close and use Pulverize first rather than WQ comboing and using Headbutt as the gap closer. By doing it this way, you can then position your victim where you want by walking behind them and bumping them forward. You'll also deal a lot more damage because your passive and hence Liandry's will be up longer and you can get 2 sheen procs instead of 1.

Utility masteries reduce both summoner skill's cds substantially. Distortion reduces the cds further to the point where people will wonder how the fudge you always have your summoner skills up.

There is really 2 items that Ali cannot go with out: Philosopher's stone and Iceborn Gauntlets

These are absolutely core to him.

Ali's mana costs are severe. So having mana regen is important. Since Ali has a slow start to his farm, the gold per 5 is also highly beneficial.

By grabbing two points in wealth, Ali can start with both a Crystalline Flask and half of the philo stone. Which half to grab depends on your opponents but the mana regen is typically better.

Ali has one of the highest base damages in the game and a short CD on triumphant roar that gets even shorter around minion waves.

This item is huge for pushing and farming. It also makes him even more disruptive in a team fight.

The mana helps as his mana costs are high. The CDR helps as its his main stat. The armor helps for him being able to tank the enemy ADC. The AP helps him deal damage and also largely mitigates poke against his team.

Everything else is open to adjustment.

A great item on him is Locket of the Iron Solari this allows him to be tankier and gives his team a good shield. If your jungler doesn't get one, you should. And honestly having 2 isn't bad.

Some really strong items on Ali include:

Ruby Sightstone take if you aren't satisfied with vision and ward coverage.
Abyssal Mask makes you more damaging and gives MR and an offensive aura for your team.
Warmog's Armor or Randuin's Omen make you a fatty.
Ionian Boots of Lucidity 15% CDR for cheap.
Liandry's Torment gives tons of damage, especially when combo'd with IBG's constant slows
Rylai's Crystal Scepter More CC and damage, but less hp than Warmog's Armor.

You'll want to grab as much Magic Pen as possible from runes, the bulk of your damage is magic (all but sheen procs) and since you are building lots of CDR, you'll want to abuse your base damages.

Armor seals and MR or MR/level glyphs are standard.

flat MR against AP top lanes and hybrids and MR/level against pure ad.

I run 9/0/21 on Ali.

I focus on grabbing all gold, CDR, and mana regen related masteries in utility and grab the standard caster 9 in offense.