The DDT is the best of all time, followed by the Stunner. Diamond cutter is a worthy one, as is the Superkick.
The best of the best finishers can come out of almost nowhere. They don't need an elaborate setup, or a guy to be already essentially knocked out prone for 30 seconds while his opponent climbs the ropes and preens. They almost look like something someone could do in an actual fight.
That's not counting submission finishers.