I'll confess, I like the classics--SPI's STARFORCE: ALPHA CENTAURI, the original Avalon Hill STARSHIP TROOPERS, SPI's BATTLEFLEET MARS, FREEDOM IN THE GALAXY, and--of course--the GDW version of IMPERIUM. Sure, there are others out there that were interesting: GDW's BELTER, GODSFIRE, SPI's STARSOLDIER, OUTREACH, as well as my favorite role playing game, GDW's TRAVELLER series and enhancements (like SNAPSHOT).
I could write fairly prolifically on each one of these, but suffice it to say they had enough wrinkles and color to keep me interested. Note I didn't list STARFLEET BATTLES on this list--while that game is really good, there were just too many versions for me to keep up (kind of like WORLD IN FLAMES!).
What are your favorites? Probably more importantly, why are they your favorites?