The top two non-boutique helmet brands are probably Arai and Shoei (in order).
My head is completely the wrong shape for the Shoei GT-Air, but I was very impressed with its feature set and build quality. If it fit my alien head, I'd have bought it.
And that's a great point for a segue into shape - figure out what shape your head is - round, intermediate oval, or long oval. Me, I'm an extra-long-oval. My dad is a round.
Bell's usual shape is the roundest brand I've found.
Shoei usual is between round and intermediate oval.
Arai goes from intermediate oval (RX series) to long oval (Signet Q)
Try lots of helmets of different shapes, and see what fits you best, and adjust sizing and fitment from there - the best helmet in the world won't do you much good if it's too loose when you need it.
-Alex
--Arai Signet Q