Here's my perspective as a native speaker from Australia:

People from the Southern US can be hard to understand. People like to make fun of George Bush's accent (which is a very stereotypical Texan accent) and say he doesn't say anything clearly, but as far as southern US accents go, his is very mild. Some people from the southern US can be very hard to understand.

African American Vernacular can be very hard to understand, but again, that depends on how strong the accent is.

Appalachian accents are <em>very</em> hard to understand. It's technically a type of Southern US accent, but it's so distinct and so much harder that I think it deserves its own section.

So we have:

-Southern US accents

-African American Vernacular

-Appalachian

The first two are only hard to understand if person speaking it has a particularly strong accent. I'm struggling to think of a fourth accent... accents in English vary much less than in other languages so most English speakers should have little problem understanding anyone else from anywhere else in the English-speaking world.

When I think of a New York accent, I think of a Brooklyn accent, which in turn reminds me of Joey Wheeler (I watched Yugioh as a kid). It's not hard to understand at all.

I think the valley girl accent is to California what Steve Irwin's accent is to Australia: very stereotypical of the region/country, but very few people actually speak like that. Nonetheless, I don't think the valley girl accent is hard to understand, it just makes your ears bleed.

I just looked up a video of Robert Frost and found this: https://www.youtube.com/watch?v=aBw-OaOWddY

This IS pretty hard to understand. I can understand virtually all of it, but I have to strain my ears to do it, and there are words and phrases which I 'miss'. Google says he was born in California to a Scottish mother and a Devonian father, and moved to Massachussetts when he was 11. His accent doesn't remind me of anything from California, nor does it sound like a Boston accent to me, but it definitely sounds like something American. It's entirely possible he has a mixed Massachussets-Californian-Scottish-Devonian accent, but I honestly have no idea. Whatever his accent is, I'd add it to the list above.

You might like to watch this video on Appalachian English:

<em>https://www.youtube.com/watch?v=03iwAY4KlIU</em>;

I already hear the banjos...