the worst:

a 3 way tie. julia roberts in "mary reilly" she sounds about as irish as tony soprano in that flick. winona ryder and keanu reeves in "bram stokers dracula", their accents (which seem to come and go at random) are completely inept and laughable.

the best (i'll stick with american accents here)

jamie bamber in "battlestar galactica". his dad is an american so that is probably why his accent is so good.

damian lewis in "band of brothers." good enough to fool a native. apparently he kept the american accent going the entire time "band of brothers" was being shot and several of the other cast members didn't realize that he was british.

dishonorable mention:

the character of peri in doctor who. i don't remember much about her back story (i really don't care for that period of doctor who) but if she is supposed to be from this side of the ATL that accent is dire.