Comfort is as subjective as anything. For full-size cans, the Beyerdynamic DT531 is my favorite. It has the softest pads around (even moreso than the 770/880), puts little pressure on the head (due to it's low weight), and has the very comfortable dual-headband system.
Why other comfortable cans aren't as quite comfortable as the DT531's:
Beyer DT770/880 (pads aren't quite as soft, headband not quite as light, higher clamping force)
various AT's with wing support system (pleather/leather pads get warm)
Sennheiser HD590/570 (harder pads, higher clamping, puts pressure onto one very small spot ont op of the head)
Sennheiser HD580/600/650 (heavier, harder pads, clamps harder)
Sennheiser HD555/595 (harder pads, feels a bit heavier or burdensome on the head)
Sony CD3000/R10 (heavier, pleather/leather pads get warm)