NES , but only for mario.
Amiga: Lemmings, Xenon, Alien Breed
SNES and Megadrive would be the real console limit i guess, but we are talking classics when it gets so old.
N64/PSX there is more titles and it's being played, just bought Pilotwings and Waverace
PS2/XBOX/Dreamcast/Gamecube got them all, the Cube has seen real action with the kid, PS2 had been played with GTA games. The XBOX is never played, as is the Dreamcast, but i plan to get some classics for the dreamcast. like crazy taxi and fighter games.
On the PC there is a mountain of old games that is still worth playing, they tend to scale better since some of them do support higher res. The only issue can be to get them to work, and in the case of multiplayer games, find servers and support for setting server up.
EDIT: Just to make it clear, i don't play through every old game, i usually just re-visit and mess around with them and is surprised by how much they suck, or how great they still are, it differs.