There is a strong consensus among pilots that the Bose X headset is the established leader for comfort, function, and quality. That lead has recently been challenged by the LIGHTSPEED Zulu.
There are minor feature differences between the two. The best explanation of these I could find is in this comprehensive review by pilot Jeff Snyder, a self-proclaimed "headset whore."
For most pilots, the feature differences will be trumped by the comfort differential, which can go either way depending on the size and shape of your coconut.
In the local pilot supply store, an older pilot swore to me that the Zulu is far more comfortable. I decided to buy them both and try them both, since they are returnable for 30 days.
Yesterday, after two hours of wear around the house, the Zulu was really putting the squeeze on. Today, after two and a half hours, I was happy as a clam in the Bose. Your mileage may vary.