So first off please read the item notes if you haven't already. They are very critical to what make this playstyle fun and interesting.

So the main premise of this build is to grab as much CDR in the runes as you can. It allows you to freely build whatever items you'd like without worrying about buying all the CDR items. Typically a tank engager support will slowly get CDR over the course of the game through basic support items. But with this set up All you need is Eye of The Aspect plus Kindlegem and you're sweet. Support items are cheap yes, but you sacrifice raw stats like health and armor for CDR and item Actives/passives. Which is not great because they're useless if you're dead.
Hexflash can be tricky to use but once you get the hang of it it's quite fun and can really catch your opponent by surprise if you're good with fog of war.

Now is aftershock and the resolves tree just better for Alistar? Yes and no haha. Aftershock doesn't FEEL that impactful. The armor and magic resist you get is very limited by time. You only get it for a brief moment and most of the time if doesn't get used fully in the early to mid game because hopefully your opponents are stunned or knocked up during its effective time. The Damage is very negligible. Its not really Alistars JOB to do damage. Is the Resolve Tree better as a whole? Yes... Yes it is xD But its not nearly as fun.
One important thing to remember is you'll be sitting at 30% CDR most of the game. Just be patient and wait for your 10% CDR from the offence portion of the rune page to kick in. By lvl 11 you'll have 35% CDR. By lvl 16 you'll have 41%. By lvl 18 you'll have 45%

Glacial Augment is a very underrated keystone for supports. Most support champions have a hard time fitting it into their playstyles. Not Alistar though. There are so many different combos you can use with Glacial Augment. So i will go through a few of them right now.

Basic Combo: W + Q + Glacial Auto into E stun auto. Glacial really helps set up for E stun and gets you into a good position.

Ali Insec: Glacial Auto(to get into position)+W+Q

Trample first: E + Glacial Auto + E stun + Q + W

Flash Insec: Flash + Q + W + E + Glacial Auto + E stun

*and More*

Pros of Playing Alistar:

Alistar’s engage is among one of the best in the game
Alistar’s abilities offer defensive and offensive options
Alistar is a tank support with sustain outside of relic shield. As of 6.6, Alistar’s heal is less effective on teamates
Alistar’s ult is one of the most effective ultimates at reducing at reducing damage. 55% at first ult rank is a substantial amount! At third ult rank, 75% damage reduction can be achieved.
Alistar’s crowd control cannot be removed easily, as pulverize and headbutt are both considered knockups.
Alistar’s abilities are simple to learn, and his influence is clear to see

Cons of Playing Alistar:

Alistar is very weak when his abilities are on cooldown.
Alistar can be left stranded because of his engage force him into the center of the battle.
Alistar has received many slight nerfs as of 8.9, a slight rework, and may change in the future.
Alistar is simple, and as such it is very clear to see what he is planning on doing. This works for enemy and ally alike. Mainly flashes or stubborn tankiness can get him through skirmishes and battles.

listar’s most powerful asset in lane is his ability to use his pulverize and headbutt offensively and defensively. Succinctly put, if one wants to disengage, you may headbutt the enemy away. If one wants to engage, you may headbutt-pulverize combo. Remember that Alistar’s headbutt changes his location as well, as such an Alistar that wishes to disengage should wait for an enemy to use their gap closing abilities before headbutting. This provides two effects; Alistar is further away from the enemy than a regular headbutt would have put him and enemy gap-closers can be interrupted. This becomes especially useful if the enemy gap-close ability used happens to do damage as-well, such as items_Le_BlancSquare Leblanc’s Distortion or items_NidaleeSquare Nidalee’s Pounce.
Remember that Headbutt allows you to choose your own fights. While it might seem obvious, do not fight battles you cannot win. Experience comes with time, but if you aren’t sure where the enemy jungler is do not engage. The best case scenario is that you win the 2v2, the worst case scenario is that you were baited and die. Unless you have your own jungler on the way or your flash is available for a quick get-away, be careful about engaging when enemies are MIA. This may be simple but I cannot stress it enough with a front-liner like Alistar. If you do choose to engage, Alistar’s most effective combo against squishies in the 2v2 is items_Flash Flash into Pulverize +Auto Attack followed by a Headbutt. Remember that the enemy can buffer their flash in some cases if you choose to Auto-Attack after Pulverize if your Headbutt is not fast enough. Be careful going in with this combo, as headbutting the enemy closer to your ADC can increase the likelihood of their death. Alistar’s goal is to successfully all in, and that means you must know enemy and allied summoner spell cool-downs. If Alistar burns a flash with a Headbutt> Pulverize combo, know that he has few gap closers left and can be kited easily. Headbutt+Pulverize is a 13 second cool-down, Flash is roughly 3 minutes in most cases. If you successfully blow a flash, back off and wait for another engage opportunity. Making the right decisions is vital as Alistar, as the wrong one can mean death.
Don't be too aggressive until you hit level 3. Avoid as much harass as possible. Go for favorable trades. Against most lanes, that would be: Q into W into E, proc your E, then back off. Don't stay too long since you basically have no pressure while your abilities are on cooldown.

Alistar is a great champion for setting up ganks. Generally, there are 2 ways to do this:

Simple W -> Q combo if enemies are in range
Flash -> Q -> E -> W enemy backwards
Flash -> W -> Q Combo to cover the greatest distance

By now, Alistar should have completed Mobility Boots and Sightstone, and have successfully been warding ahead of times on contested objectives. Warding in proper locations before fights is important for giving your team the edge, un-warded territory is likely dangerous territory. Do everything you can to avoid being caught while warding. The risk of face-checking enemy territory is not worth the reward of potentially deeper wards. Group up and push uncharted territory as a team to reduce risk. The more you stay near your teammates, the more likely you are to be able to help them. Alistar support can almost never get solo kills, the best you can hope for is catching someone out. In this case, you must hope that your team is close enough to assist you, or is getting other objectives across the map. If you ever die alone as Alistar, you have made a grave error in an earlier stage of the game. This is true for Alistar especially, as his abilities are catered to strong team-fighting.
Don't spend too much time trying to force a gank when you roam. With Boots of Mobility you won't lose too much on failed gank attempts so just head back to lane if you feel that there isn't a gank opening.

Alternatively, you can also play mind-games with the enemy. Place a Control Ward in one of the lane brushes (when your lane is being pushed) and stay in that brush for a time. Enemies may get cocky and try to all in your ADC, which is where you can use your abilities to counter-engage and maybe get a kill. You have to assess this on a case-by-case basis as this doesn't always turn out well. You can also use this tactic to bait enemies for your jungler to counter-gank.

Alistar is a devastatingly good turret diver with Unbreakable Will. Don't forget about this and make sure you start off with tanking the turret by executing your combo first!

Aim for Sightstone and Eye of the Equinox, then either look to build Knight's Vow or Righteous Glory. Righteous Glory is better when you are winning lane, as it gives even better engage potential.

The location of tanks in a late-game teamfight can be game-changing. With Alistar’s ultimate, even with his support budget, he becomes a bona-fide true tank, however temporary. The power of this cannot be underestimated. If you draw a battle-line, an Alistar who tanks the enemy back-line while peeling his own back-line is doing his job. Try not to use your Headbutt>Pulverize on the enemy back-line if your team cannot follow-up. If you prevent the enemy carries from damaging your ADC by zoning them out with your ultimate you have done your job.
Late game you will play somewhat similarly to the mid-game, however, full team fights are going to be much more prevalent. In many cases, you might not be the player who forces the engages but you also act as a very strong zoning tool for your carries. If enemies jump on your ADC, you can choose to combo them with Q -> W. If you are standing next to your ADC, you can use Pulverize, wait for the knock-up to end, then time Headbutt to maximize the time for your ADC to free-hit enemies.

Your item actives are game-changing. Righteous Glory can be used to engage or peel for your ADC. Locket of the Iron Solari should be used just as the enemy is bursting your team. Make sure to use them every team fight.

Since late-game damage is so high right now, make sure to use Unbreakable Will only when you are getting focused upon, or to break CC. Don't use Unbreakable Will if no one's aiming you, as that is a huge waste and could change the outcome of the fight later on.

Ashe is easy to deal with. Avoid her Volley harass by hiding behind minions
After using your combo, walk away and hide behind minions to avoid taking unnecessary damage
Caitlyn

Play safe and let the wave push to you
Only step on Yordle Snap Trap when Caitlyn is nowhere near
Avoid getting harassed and wait for jungle help; it's very hard to get in range without taking a lot of damage
Draven

Very difficult match-up if your ADC doesn't know how to avoid Draven's harass
Do not try to trade with Draven since you will lose most of the time
Wait for your jungler to gank as Draven is very vulnerable to CC
Ezreal

You may harass Ezreal as he is very weak in the early game
Avoid his poke from Mystic Shot and you should be fine
Try to engage on his support since Ezreal is quite slippery
Overall, Ezreal is a sub-par champion and you shouldn't have too much difficult dealing with him at all
Jhin

Avoid getting hit by Dancing Grenade by staying away from dying friendly minions
Don't be afraid to engage on Jhin as he is rather weak to all-ins
Jinx

Don't stand too near minions to avoid taking unnecessary damage
Jinx is quite easy to all-in however, take note to avoid stepping on Flame Chompers! as your return journey can be quite painful
Kalista

Due to her short range it is possible to engage on Kalista.
Kalista can dish out massive amounts of punishment on your way back and also can avoid Trample with help from her support quite easily so you need to analyse the situation accordingly
Kog'Maw

Only harass Kog'Maw after Bio-Arcane Barrage is down, otherwise you will lose trades
Kog'Maw is an easy ADC to gank, try not to push out the lane too much so that your jungle can gank
Lucian

Lucian is pretty weak now so don't be afraid to force trades
When using The Culling, body block its damage for your team if you have to
Miss Fortune

Don't stand behind dying minions as her damage from Double Up is very punishing
Interrupt Bullet Time with Headbutt if needed
Try to all-in Miss Fortune as much as you can, as she is very easy to kill and harass in lane
Sivir

Your combo is nearly instantaneous making it hard for Sivir to Spell Shield
Try to avoid Boomerang Blade during the laning phase to allow healthy engages
Sivir's On The Hunt makes it impossible to escape after a bad engage, so generally you want to engage only if you have good enough vision on the map
Tristana

Tristana is a difficult ADC to harass effectively because of Explosive Charge and her decent range
If your ADC is able to go aggressive then play aggressively along with him, otherwise play passive if your ADC scales as well as Tristana (Kog'Maw, Twitch)
Twitch

Twitch is very vulnerable to getting all-in'ed at all stages of the game, so always try to engage as long as your team is able to follow up
Varus

Varus is very vulnerable to getting all-in'ed at all stages of the game, so always try to engage as long as your team is able to follow up
Be sure to use Unbreakable Will on Chain of Corruption if you need to break its movement impairing effect
Vayne

Harass Vayne and apply pressure on her whenever possible
Exert pressure on Vayne to prevent her from getting the farm she needs to carry the game
Late-game her true damage from Silver Bolts will deal a lot of damage even through Unbreakable Will, so try to prioritize CC on her if you can
Xayah

Xayah has a respectable amount of damage early on but no escape, so be sure to keep a lookout for the number and position of her feathers when engaging
It becomes harder to engage on Xayah as time goes on due to her Featherstorm, so be aware that while she might not have a dash, she is still a very slippery ADC to deal with

Bard

Stay in positions to avoid the stun from Cosmic Binding
Bard has very high damage early on (especially if he runs Ignite) so don't be caught off guard from his burst
Bard is great at deterring aggression which makes it quite a frustrating match-up
Try to roam (although Bard will probably roam too) and snowball other lanes if possible
Blitzcrank

Stand in front of your ADC to allow him to farm easily
If Blitzcrank hooks you, use Pulverize and Trample. Save Headbutt until you need it (to escape or to disengage)
Braum

Braum is really hard to engage against because of Unbreakable and stun from his passive, which is really punishing
Your roaming ability is much better than Braum so try to abuse that fact
Janna

It's really hard to combo Janna because of Howling Gale and Monsoon stopping you in your tracks
Janna is a very passive laner so if you can push out the wave, try to roam if possible
Leona

If engaged upon, try to Headbutt the enemy ADC as Leona doesn't do much on her own
Don't engage on Leona as she has a lot of tankiness which neuters quite a bit of your aggression
Lulu

Lulu is very squishy so if your other lanes / jungler comes to help don't hold back on your aggression
Good Lulus will poke you out of lane; this is very difficult match-up to deal with so you really need to depend on your mid / jungle to help
Lulu's Wild Growth can really mess with your engages, however, it doesn't always mean that it hard counters your engages, it just makes them worse
Nami

Nami is very squishy and can be all-in'ed quite easily
You might want to take Ignite against Nami for the kill potential
You don't really want to "harass" Nami in the traditional sense as she can heal up your harass over time. Instead, you'll want to look for openings to kill her
Rakan

If Rakan tries to engage, Pulverize him and knock him backward
When trying to harass in the laning phase, try to do so when Rakan is standing next to his ADC so as to CC both of them at the same time. This is so that Rakan can't shield the damage
Rakan's healing is quite weak, however, try to avoid Gleaming Quill to reduce their sustain even further
Sona

Very squishy and easy to engage on
Take Ignite to put massive pressure on Sona
With Boots of Mobility, it's very easy to catch-up with Sona
You can nullify her ultimate with Unbreakable Will
Soraka

Make it a point to dodge as many of Soraka's Starcall to reduce her self-healing
Equinox can interrupt your all-ins. Make sure to walk out of it ASAP
Tahm Kench

TK's harass is quite difficult to deal with since his Tongue Lash is ranged
Trying to harass TK is pretty much out of the question. If you all in TK, you will almost always lose
Try to roam and make plays around the map as TK isn't as good at roaming as you are
There may be times where you may have to use your Headbutt / Pulverize combo on Tahm Kench while he has someone Devoured. This effectively CCs both targets.
Taric

Taric is quite hard to harass in lane due to his shield and healing
Try to avoid Dazzle by standing behind Taric during your harass windows
Don't get baited too hard into engages as Taric can deny those engages with Cosmic Radiance
Thresh

Thresh is quite hard to engage on due to his Flay
However, if you do get your combo off (with Trample) you can punish Thresh quite hard with your damage
Don't be afraid of getting hooked post-6. Unbreakable Will can really punish Thresh for trying to go all in
Don't engage on the enemy ADC if Thresh is standing far back, as Dark Passage will cause you to waste all of your cooldowns
Thresh can bait you into engages by giving Dark Passage to his jungler, so don't be over-eager when trying to harass Thresh unless you have proper vision