The VGChartz list of the best co-op games for every system ever.

[Dallinor]

The Ghost of RubangB said:

For Genesis: (Sega Mega Drive II)

Bonanza Bros.
Contra Hard Corp.
Gauntlet 4
Golden Axe
Golden Axe II
Gunstar Heroes (I can't believe I forgot this, it's so damn sick)
Sonic 2 (it's like 1 and a half players, and 2 players for the bonus rounds, I guess you could trade off Sonic? hey it was fun when I was like 8 or so)
Streets of Rage 2 (great beat 'em up with amazing soundtrack)
Toejam and Earl
Toejam and Earl: Panic on Funkatron

Two absolute classics. Extremely entertaining. Only Golden Axe has really stood the test of time though.