So my friends were gonna run a level 20 one shot in two days (Sunday afternoon) and I was assigned to be de facto DM. What overpowered beasts can I throw at my party (5 players I think) to make a decent challenge? These are good at playing their characters, so give me your strongest stuff

This is so open ended its hard to say but - for a group of this level I usually have the main boss have 2-3 lair skills that are on demand (at least 1 of which uses the environment ie the walls or something) ...- 1 lair skill that hits really hard that recharges on a 6 on a d6, at least 3 legendary saves, and one legendary attack skill he can do between every players round.

For a one shot I would also outline the parameters for the group IE the boss has x number of Legendary skills, X number of Lair skills, and some minions. A party of this caliber may choose to toss spells at a boss in the hopes of finding his weak save and making him burn a legendary resistance on a weak save to open him up for damage in later rounds. At least that is what my players would do. 

Really at this level - its your creativity vs the party's creativity. 

 Edit - the Legendary attack he can do between every player turn is hyper important - without that constant DPS coming in they may win in an hour, 

https://www.dndbeyond.com/monsters/4702137-hollow-knight this is a sleepy hollow monster I homebrewed. this one is a original monster I made for lower level characters but feel free to adjust how you like.https://www.dndbeyond.com/monsters/4486784-young-trenched-horror

plez let me know if you use these homebrews it would mean a lot. :)

I have (all on critterdb):
The Archydra - Like a hydra should be
The Cursed Dragon - pretty standard dragon but with a confusion breath for added chaos
The Dark Bishop - spellcaster to be used in combination with either brute creature above.
The Giant Eye - and it's minions the Eldritch Tentacle

Thunderwyvern - a proper storm dragon, use at least 2 for a party of level 20 characters. 

Or here's a foursome of horsemen that would be suitable as a single encounter for them:

Ashen Knight
Blood Knight
Black Knight
Burning Knight

Thanks guys! I'll be sure to use some of these

Why are you asking us? You're the one who makes unbalanced homebrew (rim shot)

Quote from IMPLAYS >>

Why are you asking us? You're the one who makes unbalanced homebrew (rim shot)

 Wait a minute, you're right! They'll never know what hit em! 30 tarrasques go!

You could always throw Tiamat at them! Oh_Hi_Mark has a CR 41 Tiamat that is utterly terrifying that I've been dreaming of running against my players. A beefed up Tarrasque with atomic breath would be cool too for those Godzilla lovers. 

hear me out, STAR SPAWN EMMISSARY, or Dullahan, or Any of the mythic creatures from MOoT, or, like, all four elder elementals.