Does anybody remember "Bubble" from Absolutely Fabulous? I see her name is Jane Horrocks and she had a fairly pronounced one- I will depend on others to judge whether it was good or bad.

To my ear, the trick (for a Scots' accent) is turning the "short" i into an e in words like "distillery" and, of course, the comic air leak of Rowan Atkinson's crazy people, who seem to be hissing while they build up sufficient pressure to actually speak.

I believe Robert DuVall's southern (US) accent is convincing and Chris Cooper generally does a good job. There was a minor actor in a minor film called DREAM LOVER ( which starred James Spader and Madchen Amick) who managed a quick Texas accent that sounded authentic, but soft and genteel. I think he says "Mister, she did things to me I couldn't even pronounce" but maybe that is what I would like Madchen Amick to do to me...

Christopher Guest manages a nice inoffensive southern accent in BEST IN SHOW. I think he does well with accents generally, too.

Peter Capaldi's accent in LOCAL HERO may be authentic based on his having grown up in Glasgow.