Diablo 2 and.... Mario Kart: Double Dash.

Chip 'n Dale on the NES was one of the first games I remember playing that had fun co-op.

http://www.youtube.com/watch?v=lt9aKgVKNww

FF Crystal Chronicles

Baldurs Gate Dark Alliance on Gamecube

Haven't done a lot but enjoyed an oldie called Silkworm also Resistance 2 co-op.

Honestly, I've never truly enjoyed a co-op experience. The skill set between everyone I play either makes me the frustrated teacher or the lagging loser....

Teenage Mutant Ninja Turtles: Turtle in Time
Streets of Rage 2 and 3
The Punisher
LittleBigPlanet
Killzone 3
Armored Core: 4 Answer
Final Fantasy IX
Tenchu 3 (I think that was the one with the coop missions)
Uncharted 3 coop
Spyro The Dragon (The last one. I can't remember the name)
Adventures of Batman & Robin (Genesis)
Gunstar Heroes
Dragonball Z: Battle of Z
Transformers: War for Cybertron
X-Men Legends 1 & 2
Lost Planet 2

I can't really say that any of these were more fun than the others, so I pick them all. ALso, my brother has been my co-op buddy for the last 23 or so years, so I don't know how good these games are if you don't know each other.

I just can't believe someone said this! I was about mention this game

Man, I've had a few. The entire Gears of War series, Resident Evil 5, the old school Ninja Turtles games in Arcades, Castle Crashers, a couple of Lego games, even Super Mario 3D World--but none of them compare to when my daughter and I beat The Simpsons Game. Just working together with her and seeing she was able to hold her own and solve puzzles (it really is a great co-op game) back when. I couldn't have been prouder.

By the time it came out, I didn't have anyone to play with.  I still have the game and I never got to play the co-op campaign (somehow, they fit the entire original game on the disc with co-op!).  That's one of my biggest gaming regrets.

A lot of great games already mentioned, so I am going to go older and one I haven't seen mentioned yet.  Gauntlet Legends!

This game is just packed full of arcadey fun.  One player can be a little boring at times, but it is a blast with 2 or more.