Not 100% sure if it belongs here but here goes.

Make a single house rule to break the game as much as possible. The game must still be playable in theory, regardless of how impractical it might be. So rules like: HP is calculated by Hit Die - 1000 + CON Mod won't work since it would put any character so far in the negative that they'd die immediately.

Optional: If you want, you can say how it is technically possible to play the game with the rule posted immediately above you.

I'll start: Monster AC is calculated by 10 X DEX Mod.

The Wish spell is now a cantrip, but all the negatives still remain especially the DM misinterpreting every wish to the fullest extent possible.

everybody starts off as level 20 in a game where it is supposed to be all level 1's

Everyone must have 3 as all of their stats and act like it in game. 

So basically being able to do stuff but way to stupid, weak, sick, disgusting, clutzy, and an a por head to do anything.

You add up all of your ability scores and that's how much hit points you get per level.

You can never pay in coin for anything, always having to barter for everything you're trying to buy.

(My econ teacher in high school did a project like this. We each started with a paper clip and after a month, we all bring back in what we were able to trade up for. One person brought in a tractor engine, another brought back a giant piece of polished stone, etc.)

All Martial classes start at level 20, and gain 3 Very Rare items.

(Is this necessary? 6 barbarians, for HotDQ, at level 20 with 3 Manuals. Yeah.)

Every attack is an auto hit. Your AC is just how much damage you reduce from an attack (but doesn't include the base 10, a negative dex (eg normal AC 8) would be extra damage (eg New AC -2: 2 points extra per hit)).

Quote from Swiftgale >>

Every attack is an auto hit. Your AC is just how much damage you reduce from an attack (but doesn't include the base 10, a negative dex (eg normal AC 8) would be extra damage (eg New AC -2: 2 points extra per hit)).

So... just like JRPGs on the computer?

If you say the worst haiku you can think of before casting Cure Wounds, it restores an additional 10d4 hit points, but you will be smitten by the gods of poetry for 6d4 damage.

Beastmasters get to fully control all beasts in a 30-foot radius with a reaction

Wizards can cast any spell they want (No epic spells), without any cost, but the DM is a brat and always throws the worst shit at them

The maximum age a Dungeon Master can be is six and a half.

All players must kill every NPC they meet... wait... no... that doesn't change things that much :P

your hp is your constitution score, which rises by 1d100 every level up

If exactly one PC has a Halls cough drop in their mouth, that player has 20 as all their scores until the Halls cough drop is removed by whatever means

If PCs disagree, the DM must call lightning down from the heavens and SMITE THEM!!!

Quote from ADrunkenDwarf >>

A player must have an in-game set of dice in order to make dice rolls. Rolling dice takes an action.

 But then you wouldn't be able to attack?

All NPC’s and PC’s must be the same sex as the GM

Quote from Purplecoop >>

 But then you wouldn't be able to attack?

 The idea is to break the game. You can still attack using Bonus actions.

Also, since I'm here I'll post another broken rule.

Casting any spell requires the caster to make a save using their spell casting ability: DC 12+ Spell level. Failure gives the Player a level of exhaustion.