Let's see...
Star Frontiers - I liked it, though didn't really care for the personal combat system (It takes 10 bullets on average to knock a person unconcious?). I did really like the space combat system though but the lack of customizability is a weakness.
Gamma World - Okay, though not a fan of the armor system (A lot of weak attacks could overwhelm power armor?)
Twilight 2000 - Liked the system
Robotech - Liked the system as well, though only one stat (PP) was important.
Spacemaster - My favorite Scifi system.
Traveller - I like (Obviously, since I am here), but static characters and the space combat system being unbalanced are flaws.
MT - I like better, as characters are less static. Space combat system is still unbalanced.
Mechwarrior - I love 2nd edition, 1st edition and 3rd have issues.
Paranoia - Great for what it is
Prime Directive - Read through, seemed cool, but never played it.
2300AD - Great setting. Space Combat system is really poor.
Star Wars (WEG) - I played for awhile and really liked. Granted, I didn't like that you had to spend XP to make ships/equipment better.