Probably Scotland. I met some Scottish people on vacation and I could hardly understand them. Of course, I'm sure they thought the same about me

I was not sure from the question whether you were referring to native English speakers or non-native speakers. The hardest non-natives to understand are usually Thai people for some reason. The hardest native English accents to understand are from North East of England, Scotland and Northern Ireland. Because were exposed to alot of American films and Australian soaps its not too difficult to understand them, I also find their pronunciation breaks down syllables. Try and take note of the way an American would say 'defence' / 'defense' and how a British person would say it and you will see what I mean.

I would have to say that as an American the hardest accents for me to understand would be some British ones like Scottish, Yorkshire or maybe even Welsh. But lol to what Steve said: they probably think the same thing about Americans! Sometimes I meet people from the southern US and they're accent is so strong I have no idea what they're saying! Same with strong New York or Boston accents.

Indian accent is very hard for chinese.

As an Australian it's hard for me to understand Scottish, Irish and Welsh even though these people are considered British. Also I can't understand Americans from the south.