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.
Stardew Valley
Human Fall Flat is pretty fun, funny and irritating to play with someone else
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.
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.