that is a really subjective question. there are so many and the fits are so different it really depends on you head. what you do need to look for is a helmet that is both DOT and Snell rated. Just DOT rated means they didn't spring for the SNELL testing. DOT is required by the Department of Transportation. SNELL testing is done voluntarily by the helmet manufacturers and they have to pay to have it done. its a more rigid test so if they have snell approval then its most likely better.
My son uses the M2R brand of helmet. They are from here in the US and have some trick looking helmets at reasonable costs. If you are in the UK then you'd probably want to go with a shoei or some other decent name brand thats avail. over there.