Getting back to the original query posed about the best/worst places for allergies. It certainly would depend on what one is allergic to but.....
I find the greatest relief right on the ocean where clean air is blowing off the water. I believe that this has to be where the beach faces miles of open water without other land masses to send airborne particulates my way. (It helps that the place I stay in is not moldy).
I lived in Newfoundland for awhile, way up near the permafrost. Although the air was very dry, it was very clean. My allergies were nonexistant during my year long stay.
There are maps available that indicate where airborne allergens are most prevalent. You can search for these on the internet or ask your allergist.