Alan Rickman (English) did both German and American accents in one of the ‘Die Hard’ series.
In ‘Frasier’ the actor playing Frasier’s Dad (John Mahoney) is English.
I have always been bowled over when hearing both John Hillerman (Higgins from Magnum PI) and Brad Dourif (Piter De Vries, Wormtongue) speak in their natural broad American accents. Even as Chucky, Dourif sounds nothing like he does in interviews.
It is by will alone he sets his accent in motion.
He sounded pretty good to me, too, most of the time. But it was interesting to hear him lose and regain the accent when he was singing in Once More, With Feeling. He handled the accent well enough in the smaller bits (“Walk Through The Fire”, and “Something To Sing About”), but it didn’t sound as if he could control his accent in his big number, “Rest In Peace”. He has a fairly flat singing voice, anyway, so the in-and-out of the accent kind of highlighted that.
Emma Thompson has a lovely speaking voice and I’ve heard her do general American (Dead Again) and French (Henry V) and other accents convincingly, but her funniest by far was on her “coming out” episode of Ellen. She plays herself- sort of- as a closeted lesbian lush who’s not nearly as concerned about being exposed as a lesbian and former soft-core porn actress as she is about being exposed as “born in Dayton, Ohio…” (delivered with a very nasal midwest voice).
CBS’s Without a Trace has two such actors–The previously-mentioned Aussie Anthony LaPaglia and Marianne Jean-Baptiste, who is English. Both seem to do New Yawk accents very well.
English actor Damien Lewis did a nice job playing the lead (Capt. Dick Winters) in the Band of Brothers miniseries.
Gillian Anderson is an interesting study here. Though American, she was raised in London, and her natural, childhood accent is English. Her family returned to the US when she was 11 and she was soon mocked out of her accent at school, and was flawlessly American by the time she started acting. Then she moved to London after “The X-Files” wrapped and went back to English; check out her upper-crust tones on “Bleak House”, currently being rerun on PBS. She did show she could put on a passable (to my ears, anyway) Northern Irish accent in “The Mighty Celt”.
Dammit, I came in here and read the whole thread just to make sure no one else posted this, and there you are, the next-to-the-last post! 
He really did an excellent job.
As for Hugh Laurie as House, I’ve seen nearly every episode, and I think he’s nearly flawless. I’ve only once caught a slight British pronunciation (other than when he’s intentionally doing a British accent). It was very, very slight – I only heard it the second or third time I saw that episode – but he pronounced “obituary” somewhere between the British and American ways. (British being, roughly, “obitchery” and American being “obitchuary.”)
No one ever thinks of Charlize Theron doing “an American accent,” but when I heard her speak in her very thickly-accented natural voice on a clip from a chat show she was on prior to coming to work in America, I did a major double-take. Whether people know it or not, Charlize does a PERFECT American accent.
Ooooh, Velvet Goldmine. Who cares if it’s an awful movie or not?
Christian Bale…Ewan McGregor…Jonathan Rhys Meyers…
Mmmmmmm…
While I cant speak for his gnome accent, I can say that his N. American (BK/ Dr. Angus) accent is waaaaaay to forced, I actually thought he was an aussie doing a N. American. Well, shows what I know.
Oh well, for my part I submit Helena Bonham Carter, who’s accent in Fight Club was really good, if a pit generic. I didn’t realize until I heard her speaking on the commentary a year or so later that she was English.
As did Colin Farrell in Minority Report.
Billy Bob Thornton did a convincing voice of a rural mentally disabled person in Sling Blade. The reason I think so is that we have a friend of the family with a mentally disabled brother who also happens to live in Arkansas. I used to see him around holidays and he sounds almost identical to Thorton’s character just without so much grunting.
Sadly for the indeed perfect Welsh (any nation that produces Ioan Gruffudd and John Barrowman gets my vote), Bale considers himself English. Cite. 
One And Only Wanderers, do you have any clips of Tennant speaking in his Scots accent? I have a thing for Tennant and Scottish accents.
I think Dr. Angus is OTT on purpose. He has a character on his new show with Paul Whitehouse where he plays an obnoxious American from Ohio and he sounds flawless.
RickJay writes:
> The irony being that Mel Gibson is, in fact, American by birth. Same with Nicole
> Kidman.
In a sense Kidman is. Her parents are both Australian. Her father was doing a Ph.D. at the University of Hawaii when she was born. She and her parents lived in Honolulu for a few months after she was born and then moved to Washington, D.C. for three years while her father did a post-doc at NIH. They then moved back to Australia and she lived there for the rest of her childhood. So, unlike Gibson, the majority of her linguistic influences as a child were Australian.
I wouldn’t think so. Whatever his accent is, it isn’t Chicago, or even Chicago area.
Jamie Bamber (Lee “Apollo” Adama) from Battlestar Galactica. Wife and I were blown away when we heard him talk in his native british tongue. The guy does a flawless american accent! Now that I know, I still can’t believe he’s british.
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Aw, nobody has mentioned Renee Zellweger. I thought her british accent in the Bridget Jones movies was very convincing. In fact, I find her much more charming British, than American.
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Kate Winslet does a very decent American accent, though at times it seemed a bit forced in Eternal Sunshine of the Spotless Mind.
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Leonardo DiCaprio does a bad south African accent. My brother says he sounds like a retarded Australian…I don’t know about that. But yeah, its bad.
John Hillerman is who I came in here to mention. I said this a long time ago in another thread, but I remember being so impressed with his Texas accent in that episode of Magnum, when he was “Elmo Ziller.” I thought he was really English, and here he was, doing a better Southern accent than any American actor I’d ever heard. I had NO idea then that he was a native Texan who could do one heck of a great British accent!