Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build will not make you the tankiest Alistar out there by far, I've seen alistars that literally were invincible. You will however be flipping tough.
The main point about this alistar is that you will make as much difference to team fights as the carry or the nuke. You are able to protect your team like a flipping GOD. You can send that fed yi who decided to turn twitch into minced rat flying away, before landing a stun that ensures aforementioned twitch a triple kill. All the time you will constantly be healing your team. If the enemy team decides that you do need to die you can just pop your ult and laugh as tryndamere goes from 500+ crits to 125+ crits.
Marks: Pen marks are standard for everyone, personally I think these help more than Apen, but if you like buying items such as sheen then go for them.
Seals: Alistar has MAJOR mana problems throughout early-mid game. These really help with that, they won't solve it but they will make a massive difference.
Glyphs: Enough to max out your CDR at finished build, with a little extra early-game AP to make your heals do that little bit extra.
Quints: Some would argue that HP is better, however AP is there for a VERY good reason. +15AP means that your heal will give you an extra 6. Doesn't sound like much but if you use it 10 or so times then it's as much extra health as a HP quint, and it gives the extra damage on all your other abilities aswell. +15 AP > +100 HP.
Masteries & Summoner Spells
The 21 defence should be obvious. The 5% reduction even stacks with his ult. (for 76.25% reduction in total).
9 in utility if there for the XP % summoner spells. Why only 1 in preservence? Because from 0->1 in preservence you get +2%, any extra points only give you half as much. So I stick them in the death-timer reduction as it is just more helpful. Your a tank so it's your job to stay in the fight to the bitter end and take one for the team. You WILL die eventually, the reduction is helpful.
The summoner spells are:
Clarity because it will allow you to lane a LOT longer and will help keep any ally in lane as well. It gives you another thing you can do as a support and late game can help stop you being oom when you need your stun. I have tried everything else and nothing is quite as valuble as clarity is, flash comes 2nd best.
Teleport because we want to make sure we can lane as effectively as possible, you need to be between the enemy and your lane partner all the time, keep him safe and let him rank up all the gold he needs for his much more important item build than you. If you run out of mana or HP you cannot do this, tele makes sure you are only out of lane for as short as possible. The other important reason is that Ali is one of if not THE BEST pusher in the entire game. If the opposition leave an unattended minion wave you can port in there and pulverise that tower. You can constantly heal your minions and do loads of damage to towers. If you know you can finish it and run out of minions then your ult will let you finish it off for the +dmg and reduction. Finally it allows you to be where you need to be, when you need to. Between your carry and the enemy. Teleport is FANTASTIC for Alistar.
Optional: Flash. It will help with positioning of headbut and your stun. It works really well but personally I think clarity gives a much better benefit.
We start with a regrowth and a mana pot because it will allow us to lane very easily. Once you have 715 gold, go back and buy boots of speed and upgrade to a philosophers. Then tele back in and keep laning. The next time you go back should be at 1.2k gold to get your sheen. Once you have it you gain a massive damage boost.
After that you should basically buy items whenever teleport is ready or your very low and need a heal.
Boots of swiftness: Alistar needs positioning for his q AND w, speed boost is VITAL for him.
Aegis: It increases your survivability dramatically and helps sustain nearby allies. Get it unless your other tank is getting it. If they are then I usually buy a GA as my last item instead.
Soul Shroud: It's the support bread and butter. Fantastic aura, it even increases your survivability.
Spirit Visage: Increased MR nice, CDR very nice, boosted healing fantastic. It even gievs you MOAR HP!!!! All at an awesome price. A must-have item.
Trinity Force: Mana, HP, Movement speed, Attack speed, on-hit slow and a massive damage boost. This item will help you to push towers and increase your damage out-put massively. Why not get it?
Shurelya's Reverie: It builds from out early game philosophers stone. Everything it gives is fantastic including enough CDR to reach the 40% cap. The active is also amazing at helping get your headbut in position.
Thornmail: Get against champs like tyrundamere/yi/ashe. A good substitute for Aegis.
Sunfire Cape: Same as above, which you get is personaly preference. Thornmail is better against the ranged teams while this is for the closer combat ones.
FoN: If they are almost a full AP team then consider getting this as a swap for your Aegis. The movement is also really nice and your high HP make the regen very effective.
Initiate by walking into the enemy team. If you start to take a fair amount of damage then activate your ult. When they all get close enough pop them up. Headbut anyone that gets behind you/close to your carry. Keep healing, keep protecting, keep winning.