For probably the lone Canadian entry, Dave Thomas’s Brit accent slipped quite a bit in “Arrested Development”.
Since we covered accents by Brits trying to be Americans…watch Gabriel Byrne in This Usual Suspects scene
I think I had an easier time understanding Fenster at some points.
There is certainly a standard American accent. It’s what you hear in movies and most tv news people. It is the most easily understood accent of all English speakers and regardless of where you’re from if you speak English you can easily understand it. I also am not sure that there are more variations on the American standard than on the British standard.
I hear a lot of American-accented English from foreigners, and Americanisms like ‘flashlight’.
One of the reasons why Radio Moscow’s [1980s] English language broadcasts never had much credibility here was that most of their announcers had heavy American accents.
FWIW, I worked with one of their announcers in the '90s. He was born in Washington, DC, and lived there until he was 14 or so. His dad was a correspondent for Pravda.
Beats the hell out of carrying a “torch” around with you! 
Philip Glenister’s character in a show called Demons was supposedly American. I say supposedly because he used the same exact accent he used in Ashes to Ashes, and until someone mentioned he was “American” there’s no way to infer that about his character at all. Even the character’s name, Rupert, is almost unheard of in America.
To counter all this talk about the bad Moon family accents on Frasier, I would like to point out that John Mahoney (Martin Crane) is British. He’s not really attempting an American accent so much as he actually talked that way by that point, but I still thought he should be mentioned.
A lot more Scottish than Egyptian.
The UK programme Spooks (shown in the US as MI-5) had an Irish actress playing an American CIA agent. It’s been a few years, but I remember her accent being really bad. (On the other hand the show was really good.)
Edited to add, but Demons was not good.
I forgot that Stephen Graham was English watching him portray Al Capone in Boardwalk Empire.
He had a Irish coach for a Cockney accent, and oddly, during the entire filming, no one said a word. And, the man is a legend, so I give him a break.
Costner is the winner here.
Oddly, even tho she is British (British/American, anyway) I hate Hayley Atwell’s British accent she affects in Agent Carter. Somehow it sounds fake.
Her American voice is better.
I heard her say that it was because she had been living in the UK for a while and it slipped out without her noticing or having any power over it. Nobody ever said she was a good, much less great, actress. As I understand it, this is despite her being solidly in the Method camp, but how many chances do you have to blow Warren Beatty? The man’s only human!
That’s because House’s accent isn’t real, just a generic American, that he adopted to cover up that he was raised by R Lee Ermey.
Daphne had a Mancunian voice coach on the set, but John Mahoney chose to lose his accent when he joined the U S Army. Time at Quincy and WIU didn’t help. (Go Western! Fuck Rauner!)
Ahem. Sorry for bringing politics in. Especially since I went to Northern. :o
Gillian Anderson qualifies both as an American doing British and a Brit doing American.
I had no idea that David Harewood, who plays Hank Henshaw/J’onn J’onnz on Supergirl is British. I think he does a pretty good American accent.
Dinklage, by miles. It hurts to listen to him strain so hard.
It’s British actors playing British characters, but the Weasley family (in the Harry Potter movies) has a ridiculous variety of accents for a family raised together in the same village all their lives.
Off topic, but apropos of this post: Linguist John McWhorter observed that, on The Cosby Show, the sibling played by Lisa Bonet did not speak with a Black American accent, as the others did.
One can forgive Richard the Lionheart a funny accent inasmuch as he did probably not speak English. He spoke French and Occitan. So sure he would have struggled with the language…
Rose is American but at the time the movie takes place seems to have been in England for some time, and being only 17, a mixed accent is actually appropriate.