I would probably have to say Moldorm from A Link to the Past. Got pushed off the ledge so many times.
Also, Barba from Adventure of Link is a contestant, because you have to equip Jump just to reach him, downthrust his head over several pits of lava, and his fire that he shoots at you will knock you back (and probably into the lava) if it hits you. One mess up, and you've probably fallen into the lava.