For me the worst actors are the ones that have just one presence. They are the same no matter what role they are cast in - usually just themselves.
Arnold Schwartzejager, er, Spitzennoogle, er, The Arnold
Actor formerly known as the Rock
Most martial arts "actors"
Jack Nicholson
Nicholas Cage
Ergo, the opposite would be a good actor "THAT was ____?" Johnny Depp come to mind.