Most comfortable headphones

I own and would highly recommend the Hifiman HE400S you can get on Amazon (Gramophone - same place I bought mine) for $249. They are very comfortable over long listening sessions and are pretty lightweight (especially for planar ‘phones) that also aids in comfort.  Their sound is neutral with excellent detail but avoids sounding harsh, and they can even be driven by a phone but will scale up with a better amp/DAC if you ever want to upgrade.  The main downsides are they are an open-back design that gives them an expansive soundstage, but sound does leak out of them if that’s a consideration.  Also, they are a little funky looking due to the headband, but that’s also what gives them some of their comfort factor.  Other than those caveats I’d highly recommend them for comfort, low power requirements, and sound that punches well above their price point.  Hope this helps, and best of luck. 

My partner stays up all night listening to the tv and music using Sennheiser 600’s. She has had the same pair for 20 years... we have an extra pair in case these wear out. She frequently ends up sleeping with them on. Kind of the definition of comfortable. The new 660s have the same form, although sonically have been upgraded are $499. Sound is outstanding.

Sennheiser 6xx. I have a dozen headphones, these are the most comfortable by far and sound great.