NES: the Marios, Zeldas, Megamans, Castlevanias, FF, Wizards and Warriors, Ninja Gaiden, Metroid, Shinobi, A Boy And His Blob... and obviously you have all these versions of Tetris, Dr Mario, etc.
SNES: DKCs, SMW and Allstars, LttP, FF4-6, Street Fighter 2, Yoshi's Island, Secret of Mana, Lemmings, Earthbound, Mario RPG...
N64: Mario64, LoZ: OoT+MM, Banjo Kazooie/Tooie, MK64, SSB, Goldeneye, Lylat Wars, Mystical Ninja
GC: LoZ: WW+TP, SSBM, MKDD... not sure if the Shenmue games were ported for GC, if so then you should get them
GB: SML 1&2, Zeldas, Tetris, Dr Mario, pkmn, GW Gallery, ...
out of the top of my head