You'll get lots of opinions, but the best thing you can do is try on different brands. See what fits your head the best, and is the most comfortable. Snell rated helmets supposedly comply to higher standards. I like Shoei, and Arai. Arai actually has three different internal shapes too, not just different sizes.
Also, once you find what you like, shop around for the best price. Helmet prices vary dramatically for the same make/model from different distributors.
Aria!
I found a new 'Hayabusa' blue and white RF1000 helmet on eBay last night in my size for $199
Woot woot