I've just watched Martin Scorcese's 'The Departed'. Very good film, but Ray Winstone's american accent is without a doubt the worst I've ever heard. I'm amazed Scorcese allowed it into the final cut. He's even worse than Hugh Laurie in 'House'.
I think on the whole american actors do british accents better than the other way round.
Has anyone heard a worse american accent by a non-american actor?