How can you have just one? How can you compare them?
Tie Fighter, XvT and X-Wing Alliance are some of the greatest space sims to ever grace a computer.
Dark Forces brought FPS to Star Wars! Jedi Outcast was a transcendent experience when it came out - the dismemberment! The lightsaber trails!
KOTOR is an absolutely classic Star Wars story in the same style as the OT, and KOTOR2 is a fantastic deconstruction of classic Star Wars, both with well written and memorable characters.
OG Battlefront and Battlefront 2 brought us some awesome squad shooter goodness!
Jedi Outcast and Survivor giving us some awesome modern Star Wars.
Rogue Squadron, Empire At War, Republic Commando, Episode 1 Racer, even the two MMOs Star Wars Galaxies and SWTOR, all solid games. There's no way I could pick just one.