The Daily Telegraph recently published a list of the worst 'foreign' accents attempted in films. I don't know who compiled the list, but I don't think anyone could argue with number 1 on the list -- Dick Van Dyke's supposedly Cockney accent in Mary Poppins. Van Dyke blamed an Irish voice coach, bizarrely.
Next on the list is Russell Crowe's attempt at portraying Robin Hood. His odd accent was big news at the time the film came out and I blogged on it here.
Nicolas Cage's Italian accent in Captain Corelli's Mandolin, Mel Gibson's Scottish accent in Braveheart, and Tom Cruise's and Nicole Kidman's Irish accent in Far and Away are all in the list. Angelina Jolie is on the list for her performance in Alexander, and, coincidentally, she has been criticised in the press this week for the English accent she adopts in her latest film The Tourist (eg OK calls it 'dodgy').
The Telegraph article mentioned above has links to video clips where you can judge the accents for yourself.