What are your hidden gems co-op (local or not) Steam games ?

Dead Space 3
Dying Light
Grim Dawn
Divinity: Original Sin
Divinity: Original Sin 2

local coop:
ms splosion man
ibb and obb

Trine Series
Moving Out
Overcooked 1+2
Lovers in a dangerous spacetime
Mount your friends
Ultimate Chicken Horse
And that Sonic Racing game

I played all those games quite often with friends on local coop. We had a lot of fun.

for local co-op
most of these i have played with remote local co-op
Stikbold
brawhalla-Free
towerfall
crawl
thief town
crypt of the necro dancer-if you both hate each other
duck game
most jackbox games
goat simulator
a hat in time
guacameele
most lego games
lovers in dangerous space time
mercenary kings
broforce
monaco
speedrunners

May or may not be "hidden" exactly, but these are a few I enjoyed playing with my friends:
A Hat in Time
Dungeon of the Endless
Human: Fall Flat
Orcs Must Die! 2
Out of Space
Overcooked! 2
Phasmophobia
Remnant: From the Ashes
Sanctum 2
Stardew Valley
The Escapists 2
Trine (4 games in the series)
We Were Here (3 games in the series)

i play totally reliable delivery service and animal crossing with my girlfriend and we laugh a lot,but the first game damage so much buttons (require hold on long time them) and is not confort to play with that mecanic of gameplay it's a shame because if only the dev add an option for push 1 time the button was better.

Clandestine.
asymmetric gameplay, gameplay which can be a little rought but really, really fun.
Also asymmetric gameplay the We Were Here (first one is free).
However all those games can only be played with 2 players.

Heave Ho, a ton of fun especially in local.
Blood & Bacon maybe. Not reallly my cup of tea but the game has very good reviews and is dirt cheap.
Multiplayer mods for the Just Cause can be really fun (don't justify to buy the game just for this, maybe the second often around 2.5€).
Monaco, minimalist infiltration game, quite fun in multi.
Unepic also.

speed brawl is pretty fun, I haven't tried the co-op yet but the single player is fun so I assume the co-op is the same

UNLOVED

Chariot!

Tricky Towers is good as well.

Mother Russia Bleeds
Distrust: Polar Survival
SYNTHETIK: Legion Rising
DyingLight

thare's been a bunch of great recommendations already so I'll try to skip titles already mentioned, but those are some games I played in co-op and had a blast:

Dead Island
Borderlands (entire series)
Castle Crushers
Magicka
Terraria
Saint's Row The Third
Payday (both "the Heist" and it's sequel, "2")

And many more I don't remember off the top of my head. Might add more later if I something comes up.

games i play or played with friends are

saints row 3&4
sniper elite 4
hero siege
payday 2
terraria
wreckfest
resident evil 5&6
phasmophobia
ultimate chicken horse
pummel party
tabletop simulator
space engineer
stardew valley
dont starve together
worms clan wars & ultimate mayhem 2
golf it
broforce
spelunky 1 & 2
castle crashers
divinity original sin 2
flatout 2 & ultimate carnage
a hat in time (finished it with a friend in via remote play)

games i will lokk into
cuphead
earth defense force 4.1 and/or 5

Borderlands 2

Easily 1000 hours if you are into it.

There are streamers playing this game for 4 years

Can’t believe no ones mentioned Duck Game.

A little game I enjoyed playing with my girlfriend was:

39 Days to Mars

The Adventures of Cookie & Cream on PS2!

Square Heroes

ADIOS Amigos!

Super Button Soccer

Full Metal Furies
Very enjoyable and challenging. You can check my review for more info. ;)

Games that have already been mentioned but deserve another:
Borderlands
Magika
Overcooked
Lovers in a dangerous spacetime
Earth Defense Force

Games that are more in line with the "hidden gems" subject:
100% Orange Jiuce
Killer Queen Black
Full Metal Furies

Honorable Mention:
Dark Souls (if you guys have already been playing Monster Hunter, I'd be surprised if this wasn't on your radar already, but it hasn't been mentioned yet so here it is)

Not exactly "hidden gems" but I had fun playing these:

Aragami (Dlc has some bugs but it's playable)
A Way Out
Dead Space 3
For the King
Ibb and Obb
Outland
Portal 2
Resident Evil 5 and 6 (I liked 5 more because you could share ammo)
We Were Here saga

Dyo (free)
Manual Samuel
Eon altar

Deep Rock Galactic - best and most unique coop shooter available currently imo.

Sven Co-op - free on Steam.
You can play Half-Life in co-op mode together. You can play the HL singleplayer campaign and many other great maps. Host your own server or join an online server.

Not free:
Cuphead
Portal 2
Borderlands
Castle crashers

Human Fall Flat is sooo fun with friends. I'm not a big fan of games with that kind pf physics but boy I had fun playing it. I bought it almost full price and I don't regret any single cent even though I'm the kind of person to just buy things on sale.

I have way more suggestions but they are almost all duplicates of the comments above so I'll recommend:

Shiftlings
Robot Roller-Derby Disco Dodgeball
Letal League
Streets of Fury EX