Alistair has been used primarily as Support (a very good one) by Madlife, Aphromoo etc. He is an amazing champion with a lot of potential, but what people forget is that he can be built both as an Ability Power Carry/Bruiser or an Attack Damage Carry/Bruiser (We're not talking Duo Bottom-Lane ADC, just Top Lane). He has strong AP Scaling, but his ultimate also allows him to be built as an AD bruiser that people don't know how to handle, they can truly not milk him.

The quick build is above, however I will go in detail about every item, mastery & runes below and I recommend everyone to read it.

The Runes for Alistair is as following:

As for Quintessences we go Flat Attack Damage (Flat means non-scaling runes) for more overall damage & power.

We go Flat Attack Damage Marks for overall damage & power.

The Seals are a bit different than most Top Laners and this is because Alistairs Ultimate is such a unique spell. It reduces all incoming damage by a stunning 70%, and this means he does more with Health rather than Armor. The Armor is for surviving the beginning of the game, but the health gives you the same ability + it's even better later.

As for Glyphs I go 4 Flat & 5 Scaling Magic Resist Glyphs. You generally don't play against AP Top Laners (in which case Flat MR is a lot better) and therefore you can go Scaling MR instead, although there are AP Junglers such as Elise & Evelynn, so having some Flat will come in handy for you.

Trinity Force is your first & strongest item. The Attack Speed, Movement Speed from Phage, 200% Bonus AD from Sheen is something that makes Alistair a monster.
It should be built in the order of Phage -> Sheen -> Zeal -> Completed, with a pair of boots in there. Some might argue that Sheen is stronger than Phage, and whilst it gives you a bonus in damage, you're not building AP which makes it a bit weaker than it could've been, the Phage however gives you both Health & AD Damage.

Your second item is Statikk Shiv. The trick with Statikk Shiv is that once you headbutt you can instantly auto-attack (whilst they're flying) and proc the Statikk Shiv, then back out. This allows you to deny CS by knocking them away in addition to dealing tons of damage! Another thing to note is that the auto-attack that follows the headbutt will also proc the Trinity Force, so you will do 200% more Attack Damage on the hit in addition to the Statikk Shiv.

The third item is a bit depending on the situation. If your team is doing very good and the enemy doesn't have a strong ADC or someone who can completely burst you (Zed, Talon etc.) then you can go for the Blade of The Ruined King, but if they are doing mediocre or well, go for Frozen Heart. This gives you a bit of Mana, Armor, Reduces Attack Speed of nearby enemies (which comes in handy when you go in with a headbutt) and a Cooldown Reduction which allows you to poke, heal & engage more often.

Fourth item should be Blade of The Ruined King. Although some might argue that this sounds like a terrible item for Alistair, they're quite wrong. This item alone gives you chase potential, burst damage with the active and attack speed to slap them with, it also gives you the potential to duel other tankier champions such Maokai.

Fifth item is a Randuins Omen or Spirit Visage, depending on the situation. I take it most people will understand that they need 1x Magic Resist Item if they have a strong APC in the middle or even top lane and at that point this item will do you wonder, although if they don't you really don't need it. That's why the majority of the time I go for Randuin's Omen. It gives you health which you greatly benefit from, you can do an AoE stun that rattles enemies when you engage and gives you even stronger chase potential, this & Blade of The Ruined King will mean that no one can escape you.

Sixth item is boots, although they're not bought as the final item. Buy boots when you need it badly. If they have lots of Magic Damage or CC buy Mercury Threads and if they for example have a fed ADC Ninja Tabi's is a better option. If you're just stomping I recommend Boots of Ludicity. It gives you Cooldown Reduction which makes you overall just a bigger threat with less Cooldowns to poke & engage with.

On Alistair you Max W (Headbutt) first. This is for poke Damage so that when you have Trinity Force & Statikk Shiv you have the extra damage from the spell itself.

Second we max Q (Pulverize), this is just for more damage. The damage in it self isn't the biggest when you build AD, but it is still the best option.

Third is of course the E (Triumphant Roar) which is a minor Heal, although this can be used to proc your Passive damage which is really the only thing you will use this for.

Fourth is of course your Ultimate, your by far strongest Spell. It gives Alistair 60/75/90 AD for the duration of the Ultimate depending on the level and 70% damage reduction. This makes Alistair inheritely tanky, which is why you you benefit more from Health > Armor/MR (reason for Health Seals).
It allows you to engage with W-Q with no tanky items and still be able to look like a massive tank. Once you get more health and Armor/MR you will be more or less unkillable.

Pros:
- Good exchanges with Heabutt + Sheen + Statikk Shiv
- Ultimate allows you to do a lot of damage and be very tanky, borderline unkillable.
- His Damage spikes with his first item, unlike most champions
- Gap Closer to reach the enemy backline with ease
- Can be used to peel for the ADC if you feel like it's needed
- Good 1v1 potential lategame
- Becomes a bigger threat throughout the game, never really falls off unless you're far behind


Cons:
- He is weak early game and doesn't really do that much until he gets Trinity Force.
- Can have some Mana Problems.
- Requires quite a few levels to get going
- Is a very unconvential pick (although it's seeing more popularity) so you might take some **** from people if you don't do excellent.