Ok, fair enough. I'll start with some of the PS1 games that can't be missed imho (many of these may be multiplatforms, but I put them under the PS category mainly because that was the console I had).

PS1:

Chrono Cross

Final Fantasy VII, VIII and IX

Crash Bandicoot 1, 2, 3

Crash Team Racing

Gran Turismo 1 and 2

GTA 2

Spyro 2 and 3

Marvel Vs Capcom

Driver

Tony Hawks Pro Skater 3

Castlevania Simphony of the Night

Tomb Raider 2

Metal Gear Solid

Resident Evil 1, 2 and 3

@pook99: Good list! I would add to those:

NES:

Battle City

Bomberman

Lode Runner

Yie Ar Kung Fu

Dragon Warrior III

Road Fighter

SNES:

Battletoads in Battlemaniacs

Donkey Kong series

Final Fantasy IV, V and 6

Genesis:

Gain Ground

Earthworm Jim

Multiplatforms:

Lost Vikings

Smash TV

PC:

Rayman

Master of Orion 2

Then, I don't know where the ground is with "retro". Are 32 bit consoles considered retro? Is there a year limit?