Knights of the Old Republic II. Every time I play through it, I always run into little things that I've never found before, and it's always fun tinkering with different character builds. It's possible to make a "gun jedi" that specializes in blasters, and it actually kicks ass.
Even though the ending was rushed, and KOTOR 1 was more polished, I've always found 2's story, themes, and characters to be deeply interesting and engaging.