that depends on what you like. Bangkok is extremely crowded but is more representative of Thailand. Phuket is farang Island, overloaded with foreigners.
If like beaches then Phuket. But I would choose Chiang Mai. Or Pattaya.
ok I suffer but manage with 3 air filters. Last year was the worst in over a decade. March is the worst. April can be bad if the rains come late. avoid March 1 to April 15. 2022 was not a problem
nothing to do with agriculture these are forest fires on massive scale that start mid-february. I know because I live in the middle of these fires. My close neighbor is the district fire chief and he explains it to me well. the air quality here is fine until mid February. get your facts straight