Couch co-op:

• Castle Crushers
• Trine games
• Lego games

Online co-op:

• Warframe
• Sniper Elite games (always a good thing to prepare yourself to kill nazis)
• Vermintide / Darktide
• Helldivers 2
• zombie bashing games (Left4dead, Killing Floor, World War Z, …)
• Strange Brigade (action and puzzles)
• Torchlight 2 has a co-op mod if you like Diablo
• a bunch of shooters with campaign co-op (Borderlands, Gears Of War, …)