What are the best and worst parts of the united states for living with seasonal allergies?

Varies: Different areas have different allergens—grasses, ragweed, trees, pollens, molds, mildews. Wet climates will have more molds & mildews. Warmer climates, more flowering plant allergens. Traditionally, hot, dry climates seem the best locations for those with significant allergies.

The best areas: Are coastal. Due to better air quality and constant air movement. The worse areas are those with a combination of high/heavy pollen counts in combination with high air diesel particulate matter content. Most of these are found in the southeast and southwest.