Abiotic Factor has been really fun for my buddy and me. Especially with the new update that came out last week. It’s a Half Life themed survival game.

Others that get my vote:

Valheim - Norse mythology themed survival game with Playstationesque graphics

Phasmophobia - THE ghost hunting game(see also Ghost Exile, Ghost Exorcism Inc., Forewarned)

Left 4 Dead - the original Zombie FPS series (see also Back 4 Blood, it’s kinda alright) PILLS HERE!

Risk of Rain - Pretty tough shooter series

Stardew Valley - A modern Harvest Moon, farming/life sim

Don’t Starve Together - If you played Stardew valley in hell, but everyone’s name started with a W

Factorio/Satisfactory - Resource harvesting and logistics sims. One’s isometric, ones first person, one has zerg rushes

Grounded - Honey I Shrunk The Kids: the survival game

Deep Rock Galactic - Left 4 Dead for Dwarves. ROCK AND STONE!!!

Overcooked - cooking and serving game, lots of communication required

Portal 2 - First person puzzle game, also lots of communication required

Barotrauma - Submarine sim on Europa, requires marriage levels of communication

Binding of Isaac - Roguelike shooter that’s sort of Zelda inspired, multiplayer was a little janky last time I tried it, but that was a while ago

The Forest - Excellent horror survival series

Starbound - Terraria in space

Trine series - A modern Lost Vikings, side scrolling puzzles and platforming

Subnautica 2 - A beautiful and terrifying diving/exploration game, original game has a coop mod 8 years in development, but it’s been very buggy

Diablo - First and second games are still very solid experiences and there are some excellent mods out for both

Escape Simulator - Literally an escape room simulator. Has workshop support on steam for even more puzzles.

Green Hell - The Forest, but in the jungle, much more focus on the reality of being stranded in a place where just about everything is likely to kill you.

No Man’s Sky - Space/planetary exploration sim

Dead Island - another zombie FPS

Dying Light - a zombie game with parkour

20XX/30XX - Megaman X styled platformers with roguelite elements

GTFO - Extremely hard, stealth based, alien FPS

Most “Souls” games - Very fun coop summoning, if you don’t mind the sometimes extreme difficulty

Goat Simulator series - Goofy exploration games

Magicka series - Isometric action adventure games where you combine different elements to cast spells

Barony - a true roguelike FPS RPG, voxel based, very hard

Void Crew - Space sim, mission based, sort of Egyptian mythos themed, meant for up to 4 players but definitely possible with just 2

Human Fall Flat - Puzzle/exploration game

Half Dead series - Cube: the game

Orcs Must Die series - Tower defense

Dungeon Defenders series - also tower defense, but with class based

Secret of Mana - One of the first action JRPGS, the remake has drop in coop just like the original, but I believe it’s couch coop, so if you’re not right next to each other, you’ll need something like Parsec to play it

Have a ton more, but those are the ones I can recall having the most fun. Others have probably listed a bunch of them and I probably missed a few good ones, but hopefully a few of them are new.

You could always tinker with some emulators for some retro coop games!