I almost always sleep naked, but if a situation requires that I wear something, like if I'm in someone else's house or sleeping in a room with other people (although when I had a roommate in the dorms, I slept naked,) I've found that the next best thing is gym shorts with no underwear. Boxers get twisted up, and I usually wake up with my morning wood poking out, which defeats the purpose of wearing anything, but the gyms shorts have no hole and are nice and slippery so they don't get caught in the sheets or whatever.