Games like Contra (a pain in the neck game EVEN in co-op mode AND the Konami code) are more fun with co-op mode, but beatable in single player mode.
Beat'em ups like Final Fight, Teenage Mutant Ninja Turtles, etc. are great games for co-op gameplay.
Little secret that is unknown to some people. There are a few Final Fantasy games (Main games, not spin-off games with a subtitle) that have Co-Op Gameplay.
Now the point of my rant is: While Cooperative gameplay is cool; it should NOT BE MANDATORY!! Either if its for item completion like Mortal Kombat Shaolin Monks
Oh come on! you mean to tell me that Characters AND/Or stages for VS. Mode can ONLY be obtained through Co-Op mode?
or games like Metal Gear Solid: Peace Walker where the Co-Op mode, while "Optional" it's heavily encouraged. (Translation: You will be screwed playing in single player mode.)
I don't mind encouraging players to play in Co-Op mode, but to FORCE players into playing Co-Op mode, now that I'll complain about. I will not always have a friend to partner up for these games and I shouldn't be punished for having slightly more anti-social tendencies on gaming.