Let's break Xanathar's!

[TheUser]

I'm ready to start compiling the community's list of insane XGTE builds.

I'll start with my premier build:

Ranged Gish!
Fighter 1/Hexblade 5
It combos really well afterwards with Rogue's ability to sneak attack (or College of Whispers *snort*) afterwards since the darkness spell guarantee's it and the damage scaling never slows down.

This combo allows for auto-advantage on all attacks (thanks darkness!) and can crap on bounded accuracy with an additional +3 to hit thanks to archery fighting style and improved pact weapon.

It multi-classes into some of the craziest alternate builds and doesn't mess around.
Best of all it comes online from the get go and only slightly dips behind at level 5 compared to pure classes.

The new sniper:
Gloom Stalker 5/Scout Arcane Trickster X
This will probably be the consumate ranged damage attacker by the time everything is said and done.
It acts as a perfect scout and ranged assassin with expertise in stealth and pass without trace at level 6 and expertise in 4 skills by level 8. With wisdom as a primary attribute and a likely expertise in perception you end up filling a very vital niche for going on ahead to gather intel and eliminating high priority targets. Gloom Stalker's ability to circumvent Dark Vision is bonkers. Giving a class greater invisibility at level 3 was a stupid, stupid idea. Did I mention that control flames is a somatic component only spell that puts out flames? Good bye light sources... Wood Elf Gloom Stalker can hide both indoors and outdoors with ease!

DM's take note! Devil's Sight is not Darkvision and never uses the words darkvision anywhere in it's description which will allow you to bypass this abhorrent cheese.

Caster of the Dark Gods
Divine Soul Sorcerer X/Hexblade 1 or 2
You will always start Sorcerer for Con saves but the Hex blade splash level for some bonus spells and medium armor + shields is going to save you. You may not need to get level 2 hexblade for a while but the 1/1 at the beginning is going to be key.
Twinned Healing lets you bring 2 people up with 1 bonus action. Or twin guiding bolt, or twin...SPIRITUAL WEAPON (that's right folks, it only ever targets one creature with it's attacks now it gets to target a second! WOO) Spiritual weapon has the potential to target more than one creature (subsequent rounds) and is not compatible with twinned spell.
Empowered healing will affect Enervate (since you are rolling dice to determine healing), which when coupled with Twinned Spell or Quickened Spell makes for some neat shenanigans.
The other big kickers are heighten spell with bestow curse/geas.
Taking the Dodge Action with Spirit Guardians up and proficiency in Con saves.

Using Subtle Spell with Guidance, Silence (HAH), Geas, Meld into Stone (for proper shenanigans escapes).

I want to figure out how to break College of Whispers but trying to combine it with extra attack somewhere else is proving tough. Closest I can come is a Paladin 6/ Whispers X for double smite fun.