Sci-Fi. No question. Unfortunately it seems to be one of the hardest to run successfully so GMs are significantly thinner on the ground than for fantasy.
Bushido. Again, no question. See comment above - equally revelant.
Have played Vampire (liked it, would play again), 20's Cthulhu (it was OK), Star Wars (different to Sci-Fi as there is a wild west/fantasy style to it. Liked it, would play again), Modern X-Filesian (liked it, would play again), Modern Espionage (loved it but unlikely to get to play it again).
What am I saying? I would play anything anyone wanted to run.