I don't own any super rare games. Mine are probably a toss up of the below.
Sega GaGa - Dreamcast
Blackthorne, World Series Baseball, and Pitfall - 32x
Any of the SG-1000 games.
Nothing too standout. But I have all of the rare 32X games minus Spiderman for the NA set. Then Sega GaGa and the SG-1000 games are readily found on eBay. But I don't think many exist.