The Clearest Air in the U.S. likely would be places like California, Oregon and Washington along the coast

 When its not Voggy (Volcanic Fog and dust) in Hawaii, likely the Hawaiian Islands is one of the most clear aired places on the planet too.

But, most of the time if you are not in too big a city like Los Angeles if you are anywhere on the coast from San Diego to Vancouver in Canada and beyond north likely are the clearest aired places in the U.S. on almost a daily basis.

Basically if you live most places along the west coast of the U.S. the only times the air is unclear is when there are fires and the air is in a Santa Ana Condition where it is blowing from the deserts out to sea. 

Why is this True?

Because there are around 6000 miles of open ocean to the west of the West Coast of the U.S. and there are rain storms cleaning the air all the way and particles from Asia in the air are falling into the ocean washed by the rains all this way too. So, this natural air cleaning system makes the air clear along the entire west coast of the U.S. most days of the year. 

Also, the prevailing winds are from West to East across the Pacific Ocean and onto the lands of the West Coast of the U.S. so this keeps the air clear too (except some times in Los Angeles especially during the summers).