We noticed that we are both highly allergic to Scotch Bloom this year when we stayed along the Central Oregon Coast in April/May ... other months we love Bandon, Oregon .. seems to have more open spaces and less forest areas.
There is such a diversity of greenery, and humidity around these fabulous United States that it is hard to pin down anything for sure .. but given my choices I would spend summer in Vermont, Christmas & January in Texas, and the rest of the year either in Oregon or Washington, but since that is not doable, we are looking at Idaho, Washington, and Oregon.
My daughter & I have allergies when we go to a certain park nearby, she thinks it is a specific tree (like Cottonwoods) which has a tendency to exude their seed in a packet which is like feathers .. this will stop me for sure. So we avoid that park altogether.
We love Zyrtec allergy pills for Spring & Summer in SE Washington State and Flonase nasal spray also. BUT we take 1/2 Zyrtec twice daily, and that works for us rather than one whole pill once a day. Trial and error works eventually.