i was just flicking through the channels and some straight-to-video style flintstones movie came on with that british actor who was in the full monty playing fred. easily the worst accent i've ever heard, he slips between exaggerated american and just straightup english flippantly.
natalie portman trying to do an english accent in V for vendetta was ultra stupid
i can't remember any of the top of my head but there are numerous examples of americans trying to do an australian accent and it sounding british. i think there's a reasonable amount of confusion over where one accent ends and the other begins to a lot of americans.
good: idris edba as stringer bell obviously (dominic west could possibly qualify as 'bad' but it's only when he slips out, when he's on it it sounds pretty good)