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Originally Posted by Currently  The best helmet is the one that fits you perfectly. |
Every SNELL certified helmet out there uses an EPS lining. Styrofoam. In a blunt force impact, the compression of the styrofoam is what absorbs the impact so the forces aren't transmitted to your head. We can argue the impact of shell deformation but it's still a second order effect in blunt trauma, it's the styrofoam's compression that's going to save your life.
Only two things really matter, how thick and dense the styrofoam lining is, and how much your head accelerates before the styrofoam starts compressing. To the best of my knowledge, the thickness and composition of the EPS lining in all SNELL certified helmets are approximately equal. That only leaves one thing: how much your head accelerates before the styrofoam does it's job. Get the best, tightest fitting helmet you can stand. Some of the better helmets have multiple padding thicknesses that can be varied within the same shell size to give you a better fit. The padding itself doesn't absorb the impact, that's the job of the EPS lining. But properly fitted pads will make sure that the helmet is snug but tight which allows the impact to be absorbed by the EPS lining rather than gaps in the helmet.
A good helmet fitter can help you make sure the helmet fit right and is as snug as possible without being uncomfortable. That's your best bet. Even still, it's a thin layer of styrofoam protecting your brain and it can only do so much. The scary fact is, it doesn't take a very large impact to exceed the compressibility of the liner even given a perfect fit, and once that liner is compressed nearly all the blunt force will be transmitted directly to your skull. Helmets are our only line of protection out there, but they're not a panacea. My advice: seek out the best fitting helmet you can, and realize that the best protection against head injury will always be inside your helmet.