Half way through House MD and I still can't get over how flawless Hugh Laurie's accent is.
Also thought Andrew Lincoln's was great in The Walking Dead.
Noteworthy:
Renée Zellweger in Bridget Jones.
Sean Austin in Lord of the Rings.
Alan Tudyk in A Knight's Tale.
Kate Winslet is anything she does.
David Tenant (great English accent, yes I know he's Scottish).
Will Poulter (didn't realise he was British for a long time).
Opinions and other suggestions welcome.