People said my builds were too complicated, so here's a summary. Core item order can change depending on the matchup.

Questions get me hyped

Teamfight-Oriented Builds

1. Zeke's / Sunfire / Abyssal Build

-Go for this build when your team has plenty of frontline so you don't absolutely need to engage, or when the enemy has so much frontline that you need to hold out longer or protect your team.   

Generally go Bami's -> Zeke's -> Sunfire -> Abyssal. But if the enemy has %HP damage, build Sunfire first.

2. Protobelt / Unending Despair / Spirit Visage Build

-A build for when your team lacks a frontline and you need to be the solo tank. GOAT-tier tankiness.      The order is more flexible than you'd think. If the enemy has armor shred or magic pen, go Protobelt 1st. Against AD opponents with high %HP damage, go Unending Despair 1st. If you went Unending Despair but the enemy AP is also fed, go Spirit Visage 2nd. Normally 4th item is Rocketbelt; if Zac is gigafed, go Riftmaker. For the last item, Jak'Sho or Zhonya's is recommended.

Laning Phase-Oriented Builds

1. Iceborn / Dark Seal / Spirit Visage Build

For when Zac can get an advantage (vs AD opponents), a build to increase your laning power.     Chain Vest -> Dark Seal -> Iceborn -> Spirit Visage. Around the 10~12 minute mark, if the enemy AP isn't that fed but Zac is super ahead, build Rocketbelt before Visage.

2. Abyssal / Dark Seal / Unending Despair Build

For when Zac can get an advantage (vs AP opponents), a build to increase your laning power.     Negatron Cloak -> Dark Seal -> Abyssal Mask -> Unending Despair. No matter how fed you are, don't build Rocketbelt first here.

3. Riftmaker / Spirit Visage / Unending Despair Build

A build for when Zac can get an advantage (vs tanks, armor shred, magic pen). (You can go Rocketbelt instead of Riftmaker depending on preference. Riftmaker has the higher ceiling.)     Almost always go in this order. (Even against AD opponents)

Exception 1: If the enemy top/jg/mid are all AD, build Unending Despair 2nd. Exception 2: If you went Riftmaker vs an AD laner but got dumpstered. In that case, Unending Despair 2nd.

Joker Builds

1. Jack of All Trades Frozen Heart Build

Go this when the enemy has champs like Ezreal, Kai'Sa, Jax, Sett, Xin Zhao, etc. Basically if there are many non-crit ADCs and auto-attack based champs.

The most important condition is that the enemy laner is auto-attack based (Jax, Sett, Irelia, etc.) or a tank.      Runes: Grasp, biscuits in the secondary tree, and Jack of All Trades.

1. If the enemy is Grasp/Conqueror Jax, Fleet Footwork Gnar, or a tank Spectre's Cowl -> Warden's Mail -> Boots (5 stacks) Spirit Visage -> Aether Wisp -> Mercs or Sorcs -> Sapphire Crystal (10 stacks) -> Frozen Heart -> Cosmic Drive

2. If the enemy is Lethal Tempo Jax, Sett, Irelia, etc. (high AD ratio)

Warden's Mail -> Bramble (for Irelia) -> Spectre's Cowl -> Boots (5 stacks) Frozen Heart -> Spirit Visage -> Mercs or Sorcs -> Aether Wisp (10 stacks) -> Cosmic Drive

2. Thornmail + Sunfire Build

A counter build against full AD with Blade of the Ruined King.      If the enemy is full AD and at least 2 of them are building Guinsoo's and BORK, upgrade from Ninja Tabi + Bramble to Thornmail.

During laning phase: Bami's -> Bramble -> Steelcaps -> Thornmail -> Sunfire or Bami's -> Bramble -> Warden's Mail -> Boots -> Thornmail -> Sunfire (when you need Mercs)

During teamfights, build Randuin's.

3. Glacial Augment Knight's Vow Build

A build tailor-made for a hypercarry ADC Go this when your mid is something like Galio and your support is an enchanter — a build that helps the ADC carry.       15% damage reduction for allies from Glacial Augment + Knight's Vow to take 12% of damage instead of your ADC + 10% heal from the ADC's damage amp'd to 25% by Visage + Redemption's increased healing boosts Zac's passive blob healing + E -> Approach Velocity movement speed boost + Locket provides both armor/MR and movement speed

Order: If vs AD — Locket -> Approach Velocity -> Visage -> Knight's Vow If vs AP — Locket -> Visage -> Approach Velocity -> Knight's Vow