Smash Bros. Melee and Unreal Tournament 2k4 for me, though top 10 kinda fluctuates as I forget games or new stuff comes out. Divinity: Original Sin might also peak at the top 10, as I did play that experience in co-op, and it is better with that co-op.
Overwatch might get there someday, but the bad matchmaking still holds me back. I'm sick of consistently waiting minutes to get into a game, 70% of the time it finds me a game that is missing 2-4 people, and it fails to find them in time...kicking me back to the main menu. Or I spend minutes waiting to get into a skirmish where I wait more minutes to find a game. Then when I do get into a game, my friends and I will either be matched against a team we bulldoze, or they bulldoze us. Getting a good back and forth happens too rarely for me. The custom game stuff helps, but that is a bigger crapshoot of will I end up with somebody not obscenely better or worse than me.