when it comes to comfort, i have to go with open-air designs. every closed design i have tried for any extended listening session has caused my ears to get hot or fatigued.
for me, the koss ksc-35 earclips are the most comfortable. i sleep with these on occasionally. incredibly light, and no pinching or aching feelings whatsoever. never makes my ears hot or fatigued, either. these could stay on my ears permanently, and I would not feel a thing. the successor to the ksc-35, the ksc-50, is also quite comfortable after an initial week or so of earlobe ache. however, they fit much looser and are not of the same caliber as the ksc-35 in terms of comfort (or sound).
i also find the sennheiser mx500 to be very comfortable. the foam on the earbuds alleviate any sort of pain typical of non-covered buds. i could wear the mx500 for 5+ hours and not feel a thing.
the portapros are also comfortable, but they can get a little hot and fatiguing after several hours of use.
of the closed-design headphones under $100, i'd say the mdr-v6 with the beyer dt250 velour pads is pretty light and comfortable, with very little pinching, though they do get hot after a while. without the velour pads, the mdr-v6 is supra-aural to my ears, and not very comfortable for long sessions at all. they begin irritating me after about 3-4 hours (with the velour pads). you shouldn't be driving with mdr-v6's on, however (due to noise isolation issues).
the eggo d66 were also pretty comfy, though i've only tried them for a brief period of time and can't say with certainty about having them on for long periods of time. i would estimate from my experience that they would be really comfy for up to about 2-3 hour sessions.