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well, i'm a huge gamer, so i'm tempted to give the entire list of all systems and games..but seeing as how most of the answers define "old school" as everything start-n64, i'll go with that. and since i can't decide on just one, i'm gonna answer 2 per system, where applicable.

.Atari.
Asteroids
Defender

.Arcade.
Fantasy Zone
Bonanza Bros

.NES.
Bubble Bobble
Super Mario 3

.Gameboy.
The Legend Of Zelda:Link's Awakening
Donkey Kong (1995)

.Genesis/Megadrive.
Sonic The Hedgehog 2
Toe Jam & Earl

.SNES.
Earthbound
Donkey Kong Country

.32x.
Shadow Squadron
Knuckles Chaotix

.PC.
Sonic CD
Scorched Earth

.Saturn.
NiGHTS
Fighting Vipers

.Ps1.
Castlevania:Symphony Of The Night
Metal Gear Solid

.N64.
Perfect Dark
Banjo Kazooie