Which is the most stringent bicycle helmet certification?
To test a bicycle helmet you attach it to dummy head and drop it from a height onto a solid object. The acceleration experienced by the head is measured, if the acceleration is too high the helmet fails. Stringency of the impact test depends on how high you drop the helmet from and the maximum acceleration you allow for a pass.

There's a comparison of the various standards here. Snell B95 uses the largest drop (2.2m) and is the most stringent on that count, although it allows acceleration of 300g. Other tests (e.g. Canada, Europe) have a lower limit on acceleration, but the drop heights for these standards are also lower.

If you can find a helmet that is Snell certified and also Canadian or European certified then go with that. If not, then just Snell certified is probably the best bet.