MegaTraveller
This has been my fave incarnation of Traveller ever since its release. (Fortunately for me, a LOT of other gamers agree with me and it is not too much of an issue finding MT players or games if you ask around). The best rebuild of the classic traveller system without the extensive rebuild that went into some of the later versions.
ShadowRun
Played it a bit in 1e, but got fed up with the power imbalance. Played it twice in 3e, but my gamers are all CyberPunk gamers and don't grok the SR system - nor the fantasy / cyber mix.
CyberPunk
Ahhh... CyberPunk. I won't even begin to discuss my obsession with this game other than to point you at my CyberPunk website -
http://www.ambient.on.ca/cpunk
Every Other CyberPunk Game
I've run every other CyberPunk genre game on the market, to my knowledge. If nothing else, as a chance to look for ideas we as a group could move into our CP game.
Gamma World
I'm a Gamma World junky. I *LOVE* third edition. My players really enjoy Omega World.
Star Frontiers
Played this for a few years. My first SciFi RPG. A lot of fond memories of this.
Star Wars d6
Couldn't stand it. But I blame the GM more than the system.
Star Wars d20
We like this one a lot. Especially with the revised rules. But it's no Traveller.
DragonStar
While interesting, I looked for more of a Dune feeling in the game and ended up giving up on it as published. Instead, I run a DragonStar Rokugan crossover game that we are all enjoying a LOT.
Robotech
We had a core of Robotech fans so this was a lot of fun, regardless of the system.
Paranoia
Had a few rousing games, but hard to play for more than a few sessions.