Bose Aviation X.
I value my hearing. It was a very wise investment given I was flying very noisy piston twin freighters with all the soundproofing and insulation ripped out of them. Without a headset in one of those, it was skull-shattering loud in those cockpits. Even with a (passive) headset it was still pretty loud (I also have and occasionally use a David Clark, which is a good passive headset). But nothing compares to the active noise-cancelling capability of the Bose. Lightspeed is pretty good too.
The only caveat is that without batteries (or with dead ones), the Bose Aviation X is worthless. The passive sound-dampening ability is pretty low. One of the reasons I always toted my David Clark around "just in case". Not as good as the active Bose, but I was concerned that if the Bose ever died on me or I ever forgot to change the battery, I'd still have something left of my hearing after the flight by pulling out the DC and putting it on. It wasn't as good, but it was good enough to fly more-or-less undistracted by the noise levels and preserve one's hearing.
I believe the new versions of the Bose can be hardwired into the airplane's circuitry, although I have my concerns about that too (in an emergency requiring you to shut the master off, do you really want to be dealing with uncomfortable/unfamiliar/distracting noise levels too?)
YMMV. "What's the best headset" is like all those discussions on the 911 board about "what's the best oil?"
