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!