Pioneer SE-M521 is the most comfortable headphone that i ever owned. Yes, they use synthetic leather but i am able to wear them for 5-7 hours straight with my glasses on!. They are pretty cheap in Amazon and their performance is stunning (for me)!
To be fair, I haven't used a lot of different headphones extensively. However, from the 8ish that I have now and in the past I'd say that the Senn HD 650/6XX with stock pads give me the most comfort. I've heard some complain that the clamping force is too tight, mine are not at all. I have worn them for 5+ hours many times with no discomfort. They are the newer Romanian model if that matters.