Independence Wars 1 & 2 for a little bit...I wish I had more time to get into them. Nothing has given me a better sense of the vastness of space than those two games, and the feel of the ships floating through space was more realistic than most anything else I've played.
Both Freespaces. Freespace 2 is still, imo, THE best space sim I have yet played. The action was consistently exciting, the atmosphere was very well done, the way the ships flew and felt was better than anything I've played except for perhaps the IWar games, graphics were terrific for the time, and it had an extremely immersive storyline. I play this game through at least once a year still...No other game has replicated the thrill of watching a swarm of Hornets chase down and swallow up a speeding Shivan LF, or watching a gigantic juggernaut jump into the system. The sense of awe, frenzy, and anticipation is unmatched. Unfortunately, given the way FS2 ended, at this point there is no FS3 planned, due mainly to lack of commercial success with FS2, but also in large part due to FS's developer, Volition, being sold from Interplay to THQ (the license resides with Interplay).
Starlancer was OK. It didn't compare to the production quality of FS2, and the action grew repetitive. But it was decent enough to keep me playing now and again.
Freelancer so far is repetitive and a little flat, and the planet-side experience is woefully limited, but it redeems itself with a big universe, pretty graphics, a so-far-interesting story, and fun if simplistic combat. It's unforgivably missing several elements, such as the ability to own multiple ships, but what's there is pretty well-done, and will keep me occupied for a very long time (at least until Interplay decides to do FS3).