It's so hard for me to choose a favorite, because I have so many great memories with most of these games. My "favorite" changes daily. I loved playing SWGB and JK1 way back when on MSN Gaming Zone, spent countless nights playing SWBF 1 and 2 splitscreen with my friends on the Xbox (I occasionally still do, I got my girlfriend hooked), played my first MMO (besides Runescape) with SWG , and became downright addicted to EP1: Racer.
I'll choose KOTOR today. It brought the same wonder and excitement the movies did. Its one of the first games I ever remember becoming engrossed in, and IMO it has stood the test of time. Its the RPG I hold all other RPGs to, and in a way its one of the first things I think of when I think Star Wars.