Salt & Sanctuary is a blast. It's local only, but if you use Parsec or the Steam play thingy that Valve added recently you can get it to work, though a bit laggy if you play with someone across the other side of the planet.
Dragon's Crown is pretty fun too, PS4 & PS3 only though unfortunately. You can play that locally or online as well.
One you can play Online is Grim Dawn, diablo style game with a dual class system. Pretty nice.

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I can't say they're my favorites. But here's at least 10 games I think are fun to co-op on.

Conan Exiles
ARK Survival Evolved
Minecraft
Warframe
Left 4 Dead 2
Resident Evil: Outbreak (Files 1 or 2)
PAYDAY 2
Starbound
Shadow Warrior 2
Streets of Rogue

There's more than these 10 listed games, so I'll leave it to you to search for the rest. There's a lot of good ones out there.

nice

Oh yeah, I forgot Risk of Rain 2.

It'd have to be a tie between Warframe and Risk of Rain 2 for me

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Only portal 2 ,nothing more

still good

Warframe is only fun WHEN PLAYED WITH FRIENDS. else it's just a boring, mindless grindfest. I would really love a good coop adventure that my girlfriend and I could play together. Something that isn't shooter but also not just some farming simulator.

Something like the Myst games would be nice. I know, there is Uru, but I can't get that to run anymore.

Don't have a switch yet, so this list don't have games for the console
Sonic 2
Sonic 3(& Knuckles)
Sonic Mania Plus
Transformers War for Cybertron(3 player online co-op)
Hyrule Warrior
Super Smash Bros Brawl
Super Smash Bros for Wii U
Ultimate Alliance 1 & 2 (4 player co-op)

Portal 2 was fun in co-op, but I also enjoyed Resident Evil 5 and now currently RE6. You can also give the Borderlands games a go.

nais