[Marximus]

Bose Triports and AKG K601s are at the top of my list for comfort.  The K601s have no bumps on the headband, are lightweight, and have low clamping force, so they've pretty much got everything covered.

 

[garysan]

Check out Sennheiser HD700 - probably the most comfortable headphone Senn makes. $999 I believe.

 

[fatmaggot]

i can vouch for akg's for comfort, i have a pair of 242's, sometimes i lose them and realize that they have been on my head the whole time, very comfortable.

 

[GL1TCH3D]

The hd800 is by far the most comfortable headphone I've ever used.
A used pair can go for about $900 on the low end (depends on where you live though =/)

 

[punks15]

ATH AD series is comfortable too for me