Arai all the way, simply for the superb fit on my noggin. The two best fits for me are Quantum II (which I currently use) and Vector II. The Vector is not quite as loud as the Quantum, plus the Quantum is out of production now, so the choice is clear if I need a new one anytime soon.
I literally tried on every other helmet brand I could get a hold of over the last 6 years or so, went to a lot of different stores for that. The only other brands that had some models that came close in fit were HJC and Shoei. I say close, but no cigar. I'll keep trying on other helmets as new models come out, maybe eventually I'll find an alternative. That said the Arais have served me well over the years, no complaints, and I've always felt reasonably well protected with them as well.