After recently purchasing and playing all of the X-Wing/TIE Fighter games, I have to say that X-Wing Alliance is the one that holds up the best. I don't just mean the graphics (I actually prefer the sharp, unfiltered textures of the Windows 95/DOS days to the blurry textures of the turn of the century). The gameplay is simply the best in the series. It may lack the robust multiplayer options of X-Wing vs TIE Fighter, but the skirmish mode is still fun and the story is actually one of the best from any Star Wars game that I've played.
I bought this back when it was new, but quickly realized I should have bought Unreal Tournament 2003 or waited for UT2K4 instead. There are several reasons for this: Despite coming out at the same time as UT2K3 and being a single-player only game at first, Unreal II ran like crap on my system whereas the demo of UT2K3 was smooth as butter. Unreal II is also really slow and boring. The environments are so big yet empty that it's like you're playing with some sort of miniaturization cheat active, and moving around as Dalton feels like being a dwarf in a space suit. Because of the pacing and performance issues, I never made it beyond the fifth mission. XMP supposedly improved things, but why bother when UT2K4 is available on GOG for the same price? On the plus side, the graphics are good and I'm sure that most modern systems won't have any performance issues. Sadly, none of that improves a really mundane single-player campaign. There are so many shooters on GOG that you would be better of playing (Far Cry, UT2K4, Duke Nukem 3D and especially Painkiller).