Pixeljunk Eden (this is a game where you actually have to work together to progress at all, and be constantly talking. Whereas I view alot of coop in games as just letting two people play at the same time and forcing them to work together occasionally)

LBP (just a ton of coop/competitive fun)

Rockband

Rockband/Guitar Hero is just cheating! I don't know how many hours "my band" has put into this game (those friday nights... hehe!). Great great fun indeed!

Halo:CE
Gears of War
Twisted Metal 2

1. resident evil 5
2. guitar hero world tour
3. the legend of zelda four swords adventures

Wizard of Wor 

Gladius

And Starwars Battlfront 2 are the ones i've had the most fun in off hand.

TMNT: Turtles in Time
Secret of Mana
Uncharted 2 :P (I can't think of any other so I'll go with one that I'm sure will be loads of fun)

I thought you meant Coin-Operated at first. But now I realise here they are.

3-Rise Of Nations (PC)
2-Toe Jam & Earl (Mega Drive)
1-Battlefield 2 (PC)

I can't believe so few are mentioning Halo:CE. It's Coop was epic.

Gears of War 1 & 2
Halo 3
Left 4 Dead

Baldurs Gate.

it was the longest, co-op and the reason that we had the game on hard and couldn't play solo so we were stuck on playing together until we finish the game (and the game is long)