What are the best rules set(s) you've come across for sci-fi RPGs?
Traveller d20 had the best rules I've come across so far, but they were far from perfect. The skill system was good, as was the class-based feats. And I loved a lot of what they did with the prior careers system -- though it had the inherent Traveller wonkiness (one guy has 1000 CR and a gun; another has 400kCr and a spaceship).
The thing I think I liked the most, though, was the lack of game balance. While I think it would be better if the material rewards (money, stuff) was a little more evenly distributed, I really like how characters can have different levels of skill and ability. Hell, if you want to have a character as an absolute n00b (1st level), you can do that. Or you can have a grizzled veteran. It really opens up the story possibilities, 'cause not everyone is going to have the same abilities.