I just went through this same search. My last helmet was a Shoei RF-R, which didn't fit very well (chin bar was never low enough, and always gave me a hot spot on my forehead). I have what the helmet companies call a long oval shaped head, and pretty much nothing fits.
However, I happened to land in DC the same time that the motorcycle show was in town, and I proceeded to try on helmets from every manufacturer that was there. The only two that fit properly were the Arai Profile (XXXL), and the Schuberth C3 flip up (XXXL, I think). They're both super-plush helmets (and the Schuberth has lots of neat little features), but they're both pretty expensive: in the neighborhood of $500 and $600, respectively. I ended up buying the Arai (none were available at the show, but I found it online for $450 or so) -- I'd love to try a flip-up helmet, but $600, whew. Haven't worn it on a ride yet, but after watching a couple movies while wearing it, I gotta say that it might be worth $450 for a helmet that actually fits correctly (finally).
The webbikeworld.com website has some very thorough reviews of helmets, including a special section about helmets for us big-headed types.