I am a streamer and I mostly play league of legends! I do stream some variety games as well. I was platinum on NA for seasons 5 and 6 and barely played season 6 and 7. Decided I would try this season and got to Diamond maining Alistar. I have looked over almost every variation of different alistar item paths and builds and wanted to comprise it into a guide to help others. Be sure to stop by my stream and leave me a follow! I stream later in the afternoon ~6PM EST and appreciate all the support! https://www.twitch.tv/rockstomb

Pros:
-Very strong engage / CC
-Extremely tanky with Aftershock and Unbreakable Will
-Can all in at level 2 with Headbutt Pulverize and adds additional damage and cc at level 3 with Trample
-Strong damage output all through early/mid game
-Decent sustain with Triumphant Roar and Targon's Brace

Cons:
-One of the weakest level 1 champions
-No escapes / Low base movement speed
-Does not scale as hard as some other supports

Resolve - Typical primary runes. You could try Unsealed Spellbook if you like but I stopped using it a while ago. See below inspiration notes for recommendations on what runes you would choose for spellbook.

Aftershock
Aftershock allows you to be quite tanky during your early level trades. When ccing an enemy you will receive a large amount of base resistances. It does some small additional AOE damage when you combo which can help in all ins. Guardian is not really optimal as often you can't really use it and Grasp of the Undying is a non factor.

Bone Plating
Bone Plating is good for soaking some early game poke and can allow you to get your relic stacks at level 1 easier. Other than this it will reduce three attacks normally when you are engaging for the first time. Demolish just is not necessary and Font of Life does not heal that much for it to be worth it.

Second Wind
Second Wind allows you to take more poke and sustain it back. This along with all of your Health Potion and Triumphant Roar generally means you shouldn't have to back quickly. Conditioning can be taken in a passive lane if you think you wont be getting poked Second Wind could be less useful. I have not done any testing with the new mastery Chrysalis really but the extra 60 health early seems unnecessary and the loss of it for an additional 9 AD after the takedown should not offer much.

Revitalize
Revitalize is the final mastery which works together with your Triumphant Roar, Health Potion, Relic Shield, and Second Wind. Again just allowing you to sustain all of lane without being punished normally. Unflinching can be an option if you feel you will need the tenacity but generally you won't take it. Overgrowth is pretty much a non factor.

The resolve tree is pretty cookie cutter and shouldn't really need to be changed at all normally. You can take multiple approaches for the secondary tree.

Inspiration - Inspiration will make you the Tankiest with the additional 130 health. This might be your "typical support" style build as a lot of people like to take inspiration just because they are support it seems.

Hextech Flashtraption
Hextech Flashtraption is the best option out of the first tier. It opens up different play styles and can be used dynamically. First back is typically boots so it is recommended that you do NOT get Magical Footwear. Perfect Timing also takes 10 minutes to charge up now and alistar almost never uses any Stopwatch items so neither of these options are very good.

Biscuit Delivery
Biscuit Delivery is generally a good option not for the sustain but the extra 40 base mana you get for every biscuit you use. Alistar can easily run out of mana after a few combos and biscuits can help answer that a bit. Biscuits become additionally better if you get forced out of buying Turbo Chemtank first item as you wont have the items additional mana. It could be additionally useful in extreme poke lanes but generally the sustain is not needed. Future's Market is another good option I have played with which can help you hit spikes faster like Mobi boots on your first back. Cosmic Insight does not reduce enough for it to really be worth it but you can take it if you want to save a few seconds on ult and larger cooldowns. Minion Dematerializer is only really taken for banner minions which were nerfed. Approach Velocity does not seem to have much use and finally Time Warp Tonic is mostly focusing on additional sustain which you have plenty of in your kit.

Sorcery - Alicopter likes to run sorcery with his build. He seems to currently be using Celerity and Waterwalking for roam potential and movement speed/adaptive damage. These runes are less tanky with only 65 bonus health but may provide more damage/burst through additional adaptive damage.

Celerity
Alistar is quite slow and may have trouble getting close enough to engage sometimes. Celerity provides both additional movement speed as well as adaptive damage based on your bonus movement speed. This will allow you to get in quicker and possibly do a bit more damage.

Waterwalking
The second choice for the sorcery build is Waterwalking. Alicopter seems to be running these two currently in most games. This points to a focus on movement speed/adaptive damage and thus also likely a big focus on roaming which can be important in high elo to get your team ahead. Support is a role that can flourish with movement speed and these two cater to that tendency. Another option Alicopter used previously was Scorch which was clearly for early all in damage. You could try this if you like or want to play aggressive more so in lane. The rest of the sorcery tree runes are pretty pointless and I will not cover these at this time.

Precision - Precision is another option I was seeing in some Korean and professional style builds. Again you will be a bit less tanky with only 65 bonus health but you will have some additional attack speed to get more autos off in trades and help you proc your Trample stun in some situations or get Relic Shield stacks from your greedy ADC LUL.

Triumph
Quite simply Triumph will allow you to get a large heal when killing an enemy whenever you are low on health. This can sometimes save you and also provides you with additional gold for takedowns. This gold goes into helping you get to your item spikes earlier and providing more for your team.

Legend: Tenacity
Legend: Tenacity seems to be the only other option out of this tree which will allow you to spend less time cced with some additional tenacity. This could help you land your cc sooner or period in some team fights. Again the other options are all pretty poor and I will not be covering them at this time.

Triumphant Roar

Triumphant roar is your passive. It gives you a light heal every 7 minions/epic monster/takedown and also heals nearby allies for double. This allows you some additional sustain in lane for yourself and your ADC and can provide some heals in team fights. Mostly this will help in oppressive lanes where your adc will need the extra sustain.

Pulverize

Pulverize is typically the ability you will be starting and you will also max this ability first. If you can get in range of enemies you can knock them up. In melee match ups this is a lot more possible but most lanes you will not do anything level 1. If by chance they flash in or try to all in level 1 you can hit possibly both the adc and support with a 1 second knock up for peel. This ability is great for wombo combos with your team mates and is often used after Headbutt to get in range for the knock up. Typically you can active Pulverize after your Headbutt movement begins but if you are within 200-300 units of the target you may want to give yourself some space or you may end up just headbutting them out. You can also pre-cast Q before you flash to get an instant knock up at your flash location. You do so by pressing Q immediately followed by your flash. It may take some practice so I recommend trying it out in the practice tool or in normal games. This is an important skill to master for an Alistar player and I will see if I can get a video up for reference!

Headbutt

Headbutt is your gap closer and only possible form of escape/engage and you generally will max this second. At 2 points it will have the same cooldown as your Pulverize at 5 points both being 13 seconds which allows them to always be up at the same time unless you use them separately. This ability will charge you at an enemy knocking them back. If you use Pulverize during Headbutt instead of getting knocked back your target will be knocked up as soon as you get to them. This allows you time to stack up your stun proc with Trample and is also your typical combo. If there is a minion or monster you can Headbutt to you can use it to get away from enemies but normally it will be used going in which may leave you susceptible to your enemies. It is typically best to assess where the jungler is before engaging bot lane with this ability or assessing possible targets in team fights to see who you should be ccing before using this as you may not be able to get to them without this ability up.

Trample

Trample now has a few aspects to it and typically you will max this last. It does very small AOE damage every half a second to surrounding enemies. If it damages an enemy champion you will gain a trample stack. At 5 trample stacks your next auto attack will stun the target for a second. Typically you will use this either as you combo or right after you combo with Headbutt Pulverize. It is generally pretty smart to walk slightly past your combo target to ensure if they flash you will still be in range to stack it and auto them for the stun. Alicopter tends to max this ability second with either one or two points in Headbutt. The damage the stun does is based on level so it does not seem to increase that damage and the AOE damage is increased based on points as follows: 10/12.5/15/17.5/20 base damage. This damage is pretty negligible and I suspect that he prefers having Headbutt and Pulverize on identical cooldown timers and prefers Trample to have the shorter cooldown timer: 12/11.5/11/10.5/10. It does not seem to have any major benefits but you can try this out and use it as you see fit!

Unbreakable Will

Unbreakable Will is your ultimate and you will be placing a point in this whenever available at levels 6/11/16. It reduces the base cooldown: 120/100/80 for each point and increase the damage reduction percent each point: 55%/65%/75%. This ability will cleanse Alistar of all cc effects and typically will be used when you are in the front line or to break cc if necessary. This ability can be used smartly to tower dive while receiving little to no damage. It is important to make sure you do not ult too early as you will just get blown up after and likewise you should make sure not to ult late as you may just get killed if you are too low. Try to judge what damage output will be coming your way before ulting but in most cases its better to ult early than not at all.