Midwest is the standard in American coloquials-- nice, mild, and pronounces every sylable. The Eastern such as hard New York is slowly being replaced by that, for the better, I might add, by the Midwest, and the South will always be seen as that hick-backwater area for proper speech in everywhere besides Florida. As well, Midwest is beginning to take over southern Canada, though Quebec will always have its frogs.
As for the UK, there's English accents I can deal with, but as well there's a few I can't understand (or stand) for the life of me, usually from the English armpit. Scottish is... intersting, though not neccessarilly understandable, and I have a penchant for Irish.