Wing Commander for me. A series way ahead of its time and vastly enjoyable, not just for the games themselves but for the (still very active)
community behind it. I poured entirely too many hours into playing a Wing Commander roleplaying game, for instance, not something I would do for
anything else.
Wing Commander has also spawned some of the best fan-games I've ever played. If you're a Wing Commander fan and you haven't played
Unknown Enemy, you're missing out! I hear
Standoff is pretty good too.
Not bad considering the last official Wing Commander game came out in 1998 (as an episodic Internet release - now where have we heard that idea recently...) and Origin has long since been slain by EA.