I suggest looking for exclusive releases. Road Rash II was exclusive to the Genesis. It's not a highly desired game(yet). Road Rash 1 on 3DO is superior to any other release, including the PC version and PS, so I don't usually recommend it on any other platform. I play it emulated, even though I have a FZ1 3DO in box, somewhere in my attic.
The PC versions of many games were superior, so you'll always have those titles at the prime of their glory, that's why I don't suggest some of the games mentioned. Of course, SNES was a RPG king, and if you want some good Genesis RPG's, Shinning games were very under-rated.
Bust-a-Move on SNES is the royal gem of the Buzzle-Bobble games IMO. Much like the original Tetris was king, this is still the best bubble popping game ever.
If you don't have F-Zero, something is wrong.
NES: Too many games to name. Too many different types of games. It's hard to pinpoint. I love the original Megaman series. X and X2 ect... I am NOT a fan of the later Megaman games, but I love the original. Ducktales, while easy, is worth it for the Moon level(and it's music) alone. Castlevania 1 and 3 = AWESOME. Castevania 2... eh, it's still good. Don't forget Punch-Out.
And if you want to get pissed off quick, Battletoads. TMNT1 is okay, but TMNT2("The Arcade Game") should be played through MAME, not a NES, just like other similar classics(NARC).
G/L and have fun hunting.