I don't know abut the best, but that relaxed string tune near the Valley of Mines Pass in Gothic 2 just makes me all giddy, because it's almost like a tribute to the previous game. If I could, I'd grab my sword/bow/rune tighter and start marching my head up each time I hear it in the game. Great stuff.
Tha main theme, in it's varied forms, of Toment is also very memorable. It sort of hinges between epicness and...othe stuff. Nothing like the bombastic bashing of BG 2 and IWD. Then again; IWD has some of the best single tracks I've ever heard. Like The Great Tree.