Star Wars The Old Republic. Basically KOTOR with mmo elements. You could play it as a single player game if you wanted, with a lot of story content.
But doing flash points and PvP with my friends was fun too. Buying strongholds and flagships then decorating them was fun as well.
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.
Dark Forces was one of the first FPSes to have true 3D not 2.5D. I remember specifically one mission where you had to climb on top of an elevator, and that felt awesome. Plus the way your blaster beams would bounce around the room!
Super Star Wars, Return of the Jedi on Game Boy. Not because it's a good game, far from it. It's because I'm old and have memories of trying to play this. With dying batteries and at an awkward angles so I could get light on the screen. I just accidentally found out it had this awesome Super Game Boy border too.