My daughter and I have played through quite a few games. Here are our favorites:

• ANY LEGO game, we have played through many of these
• The TRINE series. They are fun puzzle platforms
• portal 2 co-op. There is a campaign and other maps on the workshop
• brothers: a tale of two sons. Sit right next to each other and share the controller
• Unraveled 2, puzzle platform errors
• It takes two is an amazing game from EA.
• sackboy: a big adventure
• battle block theater is a fun romp
• bleep bloop
• degrees of separation, mostly fun there was a few annoying puzzles, but many can be skipped
• guacamole is a nice battle platformer
• ibb and obb is a very unique puzzle platformer
• octodad is pretty great fun
• pixel junk monsters is a tower defense style co op game
• putty pals is a fun if childish looking game
• spelunky is a rogue like
• toodee and topdee is a fun puzzler

Those are our favorites!