Star Wars: X-Wing is probably my all time favorite due to its great combination of action and strategy. A well as it's sheer volume of gameplay.
Others would be:
Dark Forces - A visceral FPS that still holds up despite its age. Jedi Knight II: Jedi Outcast - In my opinion, the definitive Jedi experience. The lightsaber fighting and force powers are the most immersive ever created, making dues very intense. It also has a lot of cool cheat codes. Knights of the Old Republic - Very engaging story, characters, and environments. The numerous choices to be made throughout adds a ton of replay value. Starfighter - A buried treasure in the library of Star Wars games. It's nothing revolutionary, but the levels are a lot of fun and the rag-tag team of heroes are enjoyable.