For me I think Wing Commander 4: Prophecy.
It's not my favorite Wing Commander game nor is it even in my top 10 list. In fact, I haven't actually played it in years. But when I think back to the summer of 1997, when I played through it, it brings back a rush of childhood memories. Like I recall that summer I discovered Imperium Galactica 2, Diablo, StarCraft, Quake, Fallout, etc. Basically it was when I got a computer of my own that was reasonably powerful for the time. Wing Commander 4 just happened to be first PC game that I played all the way through with a real story and challenge to it. At least one that made sense without a walkthrough book *cough* Myst *cough*.
First time in a game that I finished it and wished there was more. No sigh of relief that the journey was over but rather that it was finished. Heh, thinking back the live action stuff was pretty cringe but you always remember your first.