What is a good time to visit Chiang Mai and Northern Thailand?

When should I visit northern Thailand and Chiang Mai is a very FAQ; there are pros and cons for each period of the year but before we go into that, let's address a couple of common climate misconceptions, namely: 1. Chiang Mai and northern Thailand are cool (temperature-wise) and 2. The rainy season is a no-no.

View of Tha Ton during the dry season

Tha Ton, rainy season view

Chiang Mai, lying far to the north and surrounded by mountains, does have a different climate from say Bangkok and Phuket, but it's not necessarily cooler. Chiang Mai and northern Thailand have a continental climate as opposed to the maritime climate of the southern half of the country, meaning that away from the influence of the sea, temperatures are more extreme -- that is, winter's cooler, but summer's hotter.  (There! Mr Campbell, my 4th form geography teacher, would have been proud of me!)
So the winter months of November, December and January are usually nice and cool up here, but the hot season months of March, April and May can be really hot! (City temperatures can get down to 10 degrees Celsius in winter and well into the 40s in April.)
During the rainy season, from May to September, it doesn't rain every day and is unlikely to rain all day either. You can get short, sharp, refreshing showers with sunny intervals in between, and  often you can have fine days with rainy nights, so it's not necessarily a big problem.
In fact, the rainy season is one of our favourite times of year in northern Thailand. Showers keep the temperatures down, the countryside is green and lush, the birds and wildlife more active, and there are far fewer people around.
While the cool winters are a pleasant time to visit, it's peak season so it is much more crowded and prices go up. Note that in mountainous areas, the temperatures in December and January can plunge to as low as zero. Be prepared!
The dry, hot season can be pretty uncomfortable but then that's when a lot of flowers bloom and you'll have the place pretty much to yourself. You can tailor your activities to early morning and late afternoon and hole up by the pool in the middle of the day. The main disadvantage of the dry season is the smoke and dust haze, which can get very bad in certain years.