Rain world (with the downpour dlc) (incredible game)
Just shapes and beats
Crawl
Binding of Isaac (pretty sure some dlc is required)
Snipperclips plus
Duck game
Pikuniku
A hat in time (I haven’t actually played myself but my friends have)
There’s also:
Battleblock theater
Towerfall
Samurai gunn
Distance
Human fall flat
Rocket league
Ultimate chicken horse
Enter the gungeon
Almost any fighting game
You can also still play a lot of old couch co-op games through an emulator or get couch co-op mods for newer games (another commenter mentioned Minecraft). There’s a lot of options.
Castle Crashers
Untitled Goose Game
Cuphead
Unravel
Cat Quest
Nine Parchments
Any Lego game
A few of the recent Mario games
Overcooked
Diablo
Teenage mutant ninja turtles
Mario Kart (yes, switch was our main)
It takes two is great as well!
I was surprised at the sheer quality and variety of this game.
It’s fun to play with either my kid or my wife.
A couple couch co-op games i like personally are:
There’s also:
You can also still play a lot of old couch co-op games through an emulator or get couch co-op mods for newer games (another commenter mentioned Minecraft). There’s a lot of options.
Stardew Valley
Human Fall Flat is pretty fun, funny and irritating to play with someone else
The trine games! The first two are a little dated (while still being very fun) and the third one is pretty bad, but 4 or 5 are accessible and great!
Don’t know about others, but you can barely call Odyssey co-op. Cappy mostly just watches Mario play.
Wonder