From the BBC:
Dick Van Dyke is celebrating his 80th birthday today.
The rather odd Cockney accent that Van Dyke employed in Mary Poppins stands alone in cinematic history (Wikipedia says it is "still often cited as the worst attempt at a British accent by an American actor"). So as a tribute, we are inviting your impersonations….
Dick’s extraordinary performance surely is the worst filmic attempt at a British accent ever – I don’t think that can seriously be disputed. But in second place must come Keanu Reeves in Bram Stoker’s Dracula. I don’t count Kevin Costner in Robin Hood as he wisely didn’t even bother trying.
The best must be the David St Hubbins and Nigel Tufnel characters in Spinal Tap. Their Estuary English is more convincing than Mick Jagger's (though that's not necessarily saying much).
As for returning the favour, Bob Hoskins might take the prize for terrible American accents. In Who Framed Roger Rabbit? he seems to forget halfway through that he’s supposed to be a hardbitten Noo Yoik cop, becomes a hardbitten cockney DI, before belatedly remembering to be American again in the final scenes.