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?”
Here’s the explanation of the accents behind the language, the accents ranked, and then an explanation of some of the slang.
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.