BSI and ECE ratings for me.
Motorcyclist did a independent review a couple years ago that brought to light some very interesting facts. Here is a link to the article.
Many discussions have been had on this subject, some agree with the study, some still insist that Snell is the only way to go. There's a reason Snell is not approved in WSB and MotoGP.
I have been wearing AGV helmets for years, the BSI and ECE rated ones and see no reason to change.
Snell, on the other hand has printed a revamp of it's standards for 2010, I believe, and they are amazingly similar to the ECE and BSI standards.