I don't know about bad accents, though I can say most times people try to do an English/British accent it is horrible.
But good accents, I think Jude Law in I Heart Huckabees did a good job, and does usually in general. I didn't know he was British for a long time, same with Nicole Kidman in most of her movies, I had no idea she was Australian for a long long time. I'm trying to remember, there's someone else who did a really excellent American accent in some movie, it wasn't even a proper accent it was like a slimeball, loose/slang accent. Ah well
But good accents, I think Jude Law in I Heart Huckabees did a good job, and does usually in general. I didn't know he was British for a long time, same with Nicole Kidman in most of her movies, I had no idea she was Australian for a long long time. I'm trying to remember, there's someone else who did a really excellent American accent in some movie, it wasn't even a proper accent it was like a slimeball, loose/slang accent. Ah well