Paul Freeman horrible French accent as Belloq in Raiders of the Lost Ark is the one I immediately think of. He sounds nothing like French.
True. On the other hand, he was legendary in the role so I ignored that minor distraction.
He was amazing in the role. I just wish the character wouldn't have had to be French. Also I would have loved to see how Jacques Dutronc would have performed if his English didn't suck so much. Since he was Spielberg's first choice for the character.
Stephen Macht in Graveyard Shift. What I thought sounded like an attempt to do the accent of a brain damaged Eastern European turned out to be an attempt at a New England accent. It is gloriously awful.