I am pretty convinced that there IS no "quietest" helmet- there are too many variables to take into account, including head shape, bike you're riding, how you ride it, etc. You get the idea. I just find the helmet which fits the best, and wear earplugs.
I have tried the Quiet Ride System, which adds "earmuffs" which inflate to the sides of the helmet, and yes, it does work, but not to the degree that I would personally be ok with. The install is a bear: I would not do it again, but I do enjoy the lack of earplugs, and the ability to use my music. I'd estimate a 15 dB reduction in noise from them.