Which helmet?

[andym]

The American standards are a bit more demanding than the European ones, but most helmets are made to comply with both standards, so you are unlikely to go wrong there. The best helmet is the helmet that fits, and you are the only one to judge that. So go to a real shop, and try them. Personally, I need an extra large size, and the special oversized Giro GXL is the only one that will fit my big head.
Willem

There are different standards. The Snell standards are the most demanding and most manufacturers don't have their helmets tested to this standard but to the less demanding ANSI and EN standards. But I agree that a good fit is fundamental.