Honestly, there are a ton of couch Co-op games for the Switch. Obvious ones that you'll get recommended to the most are Overcooked 2, Super Smash Bros. Ultimate, Snipperclips Plus, Cuphead, and Super Mario Party. Soon, Untitled Goose Game will feature Co-op, so be on the look out for that too. There are also some less popular games that you should try out! Like indie games and such. Most of these are on sale for the appropriately named "Share the Fun Sale." I made a thread about it that you should check out, which is also pointed out by
@Sky.
Out of the ones on sale right now, I would recommend the following:
Luigi's Mansion 3 - Which you should definitely get! Playing as Luigi and Gooigi is so much fun!
Cuphead - Although the price isn't the lowest price ever, it's still really fun! Defeating the tough bosses feels even more fun when you have someone else to share that joy with.
River City Girls - It's a great beat em up game that my friend is somewhat obsessed with.
Puyo Puyo Tetris - It's a great puzzle game that I'm somewhat obsessed with.
Go Vacation - Doing the activities are even more fun when you're doing them with someone.
Heave Ho - fun and intense game that is simple to understand and gave me a lot of laughs when playing.
WHAT THE GOLF? - Chaos. Chaos is always good. (Not in real life though! But it's just a game)
These are the games I would recommend. I haven't played everything on the list, so there may be some other great titles on sale rn, especially with the indie World sale going on alongside the Share the Fun sale.
Maybe I'll make a post for that too.