For me it's English spoken by Japanese people (when they have that strong accent), because it's only understandable to other Japanese, lol... Quite often some of them looked at me and started talking in English, but I could barely understand them.

Hi metra .. Very nice topic to discuss.. Ithink most difficult accent is Indian accent .. Cuz them language is difficult to ather speakers ..

Here in Germany I learned British English first then I learned American English later on, but like you I have trouble understanding someone with an Indian accent speak English, I can understand usually if they speak a little slower but if they're fast I can't make out what they're saying, but I can completely understand my Iranian friend when he speaks English. I also  learned how to speak Spanish, the Spanish they speak in Spain and I've been to Spain and I could speak Spanish to them just fine and had no problem understanding the accent, I've also gotten better at understanding the Argentinian accent but if I talk to someone from Venzuela or Columbia sometimes I have a hard time understanding their accent, sometimes I don't but I've had times where i'm just like "Lo siento no comprendo." Also with American accents I usually don't have too much trouble understanding the American accent because we have so many Americans coming in and out of Germany that I've gotten used to how they sound, but get into the southern American states like Texas, Arkansas, Georgia, Tennesse, those areas if they don't anounciate clearly enough I can't understand them. I'm not sure if one specific accent is the hardest for me, it just depends on the accent but I'm getting better at understanding accents so hopefully this won't be an issue for very much longer.