The immortal line from the late and much-missed Dennis Farina comes to mind (speaking to some Brits when they throw some unintelligible wordslush at him):
“You guys invented the language; why don’t you fucking speak it?”
In terms of difficulty, there are only three that I find absolutely incomprehensible (in order): Glasgow, Geordie (Newcastle/Sunderland) and Liverpool.
For the record, when I’m in Britishland I tend to speak Public (a.k.a. private) School Pronunciation — after I’ve been there a while and lost my slightly-Americanized/Texas accent.
But I fail, and while I am well-spoken, my native Johannesburg wins by two lengths.