Some of the sci-fi games I've played/read:
STAR FRONTIERS - Lots of fun even if the game is rather simplistic. The Volturnus modules are still the most fun of any sci-fi modules I've ever read or played (although I recently picked up a copy of TWILIGHT'S PEAK on Ebay and I'm really looking forward to running it!).
ALTERNITY - I've never played it but I do like the system. Some of the rules seem a little more involved than necessary, but the races are really great (much better than the Aslan or Vargr but I've ranted about that on another post...
TRINITY - A comic book crossed with BABYLON 5 crossed with 2300 AD, but WW pulled it off well I thought. Good psionics rules and again, great aliens (more info on them would have been nice).
STAR WARS (WEG) - I think this is one of the most fun games ever created. The system is fast and easy and really captures the feel of the movies. A pity that they lost the license to WotC, which brings me to...
STAR WARS (WotC) - I haven't played it yet and I've only skimmed the rulebook (I got it for a song on Ebay!). It seems ok but I never really thought that d20 fit STAR WARS too well. I'll have to try it and see for myself.
WORLDS BEYOND - I originally bought it for ideas to use in a MEGETRAVELLER campaign I was going to run, only to discover I liked it much more! It had fast,simple rules and a TRAVELLER feel to it. Plus the artwork was MUCH better than anything found in any of the MEGATRAVELLER books (and I owned them all!). As a result, I sold all my MEGATRAVELLER stuff and proceeded to work on a WORLDS BEYOND campaign that I never got the chance to run. All my friends wanted to play at the time was AD&D, MERP, and Star Trek.
STAR TREK:The Role-Playing Game - The old FASA version was great (IMHO) and a favorite of my gaming group as well. I didn't think too much of the LUG version although the task system reminds me very much of MEGATRAVELLER, which I feel had a great task system and I believe should be the system used in T5 (assuming we ever see T5, which is doubtful).
GAMMA WORLD - My first sci-fi game and still on of my all time favorites. Bash it if you must, but my friends and I always had fun playing it and it will forever hold a special place in my heart.
TRAVELLER:The New Era - I enjoyed TNE's setting a lot but the rules were a nightmare to learn and use (ROLEMASTER is simplisity itself by comparison!). It took me a MONTH to figure out how to conduct space combat and the fuel consumption equations still cause me to wake up screaming!
MEGATRAVELLER - A great task system attached to a combat system more complex than necessary. Overall very good but more examples in the rules would have been nice and more published adventures would have helped a lot as well.
T4 - A good idea very poorly written. The supplements sucked. Mileu 0 could have been really fun but the written support material was terrible and boring to boot!
CLASSIC TRAVELLER - Last but not least is Classic Traveller. I owned The Traveller Book as a lad but never really read it. If only I had, I'd never had to have looked all these years for a great sci-fi game to play. You can do ANYTHING with these rules. At long last my friends and I have a really good sci-fi game to play!