Buck Rogers: Countdown to Doomsday - A complete remake of SSI's Gold Box PC RPG. It was ahead of its time, with the ability to complete even the major story missions in your preferred order, as well as choose your own path within them. I've never seen another turn-based RPG successfully combine both space and on-foot combat, which is a big deal for a sci-fi fan like myself. Also, that first mission after the prologue... just brilliant.
For the Genesis, this would be the one game I'd keep if I had to choose.
Just wanted to mention that since it's extremely obscure.
Edit: Actually, let me add a couple more, both PC ports/remakes.
Starflight - A remake of the PC space adventure game. Create a crew and explore the galaxy, gathering resources and interacting with alien races both familiar and previously unencountered. This game was pretty big. I actually never got very far, though. I stopped when I learned of the
fan crafted sequel (which I've yet to play, ironically). Anyway, another game well ahead of its time and also a source of inspiration for the Mass Effect designers.
Star Control - A port of a PC space strategy game. A number of mini-campaigns to play through using a variety of different ships that fight in real time during actual encounters. I'm having trouble remembering this one clearly, but it, too, has had a sequel, which was later remade by devoted fans into
The Ur-Quan Masters.