Republic Commando has a great single player, and also a pretty good multiplayer to back it up.

Republic Commando is truly the best Star Wars game to date. You play as Delta 38, leader of the Delta Squad. There are three other squad members that you can control around with ease, and the A.I. is amazing for them. In the single player missions, you'll find yourself on Geonosis, Kashykk (you will fight along side Wookies), and a mysterious "Ghost Ship". The story is excellent, and the game is worth it just for the single player. Multiplayer, however, is not to be forgotten. While it's not exactly Halo, there are a good balance of weapons and maps. You play as Trandoshian, or Clone, and there are multiple game types to play. Republic Commando is a solid experience through and through.