Another one for Jane Leeves as Daphne in Frasier. Fucking awful Manchester accent. I am pretty sure she is just from the South East of England. Why did she not just have THAT accent?
(Like either a bit posh or estuary accent.)
Also Leonardo DiCaprio is amazing, but his accent in Gangs of New York was awful - (he did Irish.)
Also, whilst I like Line Of Duty, I HATE the way Martin Compston speaks in it. He is putting on an estuary accent (south east England,) and it sounds so fake and put on. I just hate it. Again, why could the character not just have been from Scotland/from Glasgow (like Martin is,) and had a Scottish/Glasgow accent? Adrian Dunbar and Vicky McClure do their own accents!
Yes to Emily Blunt as Mary Poppins! Awful! Should never have been remade. Julie Andrews accent/voice is exquisite ...
And whilst I love Renee Zellwegger, and did quite enjoy the Bridget Jones films, I thought Renee's British/English accent was a bit OTT, sounded quite put on, and was way too 'posh.' Why did they not use a British/English actress? Same with anyone really. Why not use an actor or actress who has the accent of any particular given role?!
I also hated the way Helen Baxendale spoke in Friends. Why did she have to put on that stupid fake 'posh' accent?!
I agree with a pp that James Marsters as Spike in Buffy was very good!
I was stunned when I found out he was American and not British/English. (I was obsessed with Buffy!!!) 
ALSO, Gillian Anderson did a good American accent. I didn't know she was British/English til about 10 years ago. All through the X-Files (which I was obsessed with too,) I thought she was American.