Cracked.com has put together a list of what they think are the worst accents ever. The ones that come immediately to mind for me are:
Holly Hunter in Once Around
Angelina Jolie in Alexander
Don Cheadle, Oceans 11
Michael Caine, The Cider House Rules
Al Pacino in Scarface (even though it’s totally great)
Kate Winslet’s American dialect in Titanic (it just fell flat for me, I know, how dare I?)