The best one is one that fits and that you'll wear. Go to a dealer and try them on. They should be able to help you find the right fit.
The full face helmets offer the most protection from bugs, inclement weather and keep your face pretty if you happen to be unfortunate enough to land on your face. They also make it easier to identify the remains after a horrific crash.
The least protection is from beanies.
Get a helmet with vents to help keep you cool in warmer weather. Although in extreme heat, they seem to be of little benefit.
Shoei, Bell and Arai are quality brands. I try to stay away from the cheapest of the cheap.
Personnally, because I have a windshield, I wear Nolan or HJC in shorty or half helmet and add a face shield when the weather turns foul.