Ooh, that's a toughie. I guess my favorites to watch are a side one-and-three on floor and a full-twisting reverse hecht on high bar. Chris Young is my favorite to watch doing either one. I also really love double-layouts, but I don't really know who my favorite gymnast is to watch doing them.
My favorites to do myself are double backs on floor, as well as just about anything landing in a rollout.