I cannot narrow it down to only one.
Star Wars Galaxies (pre Jump to Lightspeed)
KOTOR
KOTOR2
Rogue Squadron
Rogue Squadron 2
All the Jedi Knight games
SHADOWS OF THE EMPIRE - you all disgust me for not mentioning this
Battlefront and II were both pretty decent
REPUBLIC COMMANDO - Underrated, once again you guys fail for not mentioning this.