If you are planning a visit to the highlands like Khao Yai, Mae Hong Son, Mae Salong and hills of Chiang Mai area, the best time to visit would be during year end/ beginning of the year where the weather is cold. And the most beautiful flowers would bloom.
And for highlands, if you visit from March to June, the scenery is not as breathtaking as it is the dry session. However if you are not intending to visit the highlands and wish to enjoy the tropical weather, you are most welcomed to visit Thailand anytime.
Below are some pictures taken from TSA Garden located in the highlands of Loei province in December where the weather is cold: