March Madness Non-D&D Blog Challenge: Day 10: What science fiction RPG have you enjoyed most?

Paranoia, for sure, with an honorable mention to Gamma World.  I've also played Traveller, d20 Future, a bit of Warhammer 40K Roleplay, Star Wars Roleplaying Game (WotC), Space 1889, and CORE Command.  Traveller was a long time ago and I think I'd like it a lot more if the setting was totally different.  The rules are simple (almost simplistic) but quite adequate.   I ran a scenario of Warhammer 40K and it was fun; I could definitely run more.  The Star Wars game was run by a friend and it was fun (I played a gungan spy, which was riduculous).  CORE Command was interesting (hi, Dan!) but I found the SilCORE rules hard to grokk; skills had two aspects and I just didn't quite get what that was supposed to be about.  I've run d20 Future and I rather like the d20 Modern/Future rules; actually I ran a Warhammer 40K game with them and it was fun.  But the most outright fun sci-fi game is Paranoia, hands down.  It's designed from the ground up to simply be fun.  Yes, you could run a gritty, hard-core game with the Paranoia world (and I'd like to) but the real Paranoia is totally a blast just as-is.