In the 70's - 80's Molly Sugden had a series called "That's my Boy" the boy was played by a guy called Chrisopher Blake. Blake went on to "star" in another series called Mixed Blessings. He is without doubt the worst actor I have ever seen.
The ones that get me are the over-rated actors who can't act, such as Michael Cain. Never saw him do a good accent or act any differently to his own self.
In the finest Michael Caine tradition, he has only ever acted as himself - fine for Bond, not so fine for an 'Irish' cop in 'The Untouchables' for which HE WAS GIVEN AN OSCAR!
Hugh Grant almsot always plays the foppish 'Actually I'm pretty gorgeous aren't I?' idiot.
Amy McDowell - a face you could never get tired of walking away from!