It depends on what is important to you.
In terms of safety, nearly all (full face) DOT / Snell certified helmets will protect you the same. But safety is not the only thing to look for.
For quality (noise reduction, vents, comfort),
AGV, Shoei, Arai, and Bell offer the best quality helmets.
If you ever try on an AGV Ti-tech you will notice immediately that useless outside noise is eliminated yet you can still hear horns, etc so your hearing is not hindered. The helmet is extremely comfortable and the venting system is excellent.
Shoei's RF-1000 helmet is another top quality helmet that is significantly less in price than AGV's Ti-Tech. We recently reviewed this helmet and I own and wear one myself. The RF-1000 is also excellent at noise reduction - something that is important to me personally.
For more details, our review can be found here:
http://www.alivetoride.com/motorcycle_gear_reviews_shoei_rf_1000For style -
Icon and Arai seem to coming out with the best designs in the past two years.
We reviewed Icon's Airframe helmet earlier this year which can be found at:
http://www.alivetoride.com/motorcycle_gear_reviews_icon_airframe_helmetThe designs for this helmet are numerous and are the most vividly artistic helmet designs I've seen in a while.
For price -
HJC, Scorpion and Vega offer great prices for a helmet that will protect you quite well. If you have a tight budget, or are looking for a backup / passenger helmet - HJC is definitely the way to go.