My younger cousin comes over today (like every few months) and we will be playing some couch coop games over the weekend.
So far we tried a lot of games and found some great ones along the way. Sidescrollers work sometimes, but third person and isometric is better. And I won't touch another twin stick shooter haha.
Do y'all have any suggestions, it's always so much work to find a good new game (as opposed to just going after steam review, buying, and returning within 30min of playtime because it sucked).
Honorable mentions:
Swords of Ditto (literally no one plays this, but it's a pretty neat co-op zelda-like
Darksiders Genesis (hack n slash)
It takes Two (co-op puzzle)
Children of Morta (great roguelite)
I also bought Secret of Mana, maybe we'll play this.
I'll second Portal 2, and other's mentions of Lovers in a Dangerous Spacetime.
It Takes Two is fun, but if you're paying attention to the story I found it to be a bit preachy about what the writer thinks a good relationship should be.
Smash Bros and Mario Kart are both competitive not co-op.
I play smash with my 7 year old on the same team. We cooperate as teammates to defeat the other team. I'd call that co-op. The same can be done with Mario Kart, but whatever.