According to what Thais say - Thailand has three seasons hot, hotter and hottest. So, it completely depends on what time suits you and what part of Thailand you're visiting. The best time to visit Thailand could be best explained by the region of Thailand you plan to visit.
Best Time To Visit Thailand
The best time to visit Thailand is from November to February when the monsoon wind brings cool, dry air. The northern part of Thailand
usually consists of Jungles and temperatures can climb up to 10 degree Celsius from November to February and after that it starts getting warmer.
In Bangkok/central region
the temperatures is cool and nice around 18 degree Celsius from November to February and after that the temperatures go up to 35 degrees in the month of April and May. It gets humid during the rainy season and puddles of water could be found in some places of Bangkok.
In the south of Thailand
which receives a lot of rain throughout the year the best time to visit is from November to February as it rains not as much. The maximum amount of rainfall you will receive is during the Monsoons all throughout Thailand and the humidity is high as well. The tourists in the beautiful islands of South usually come between November to February
The central Thailand usually gets dry and humid from October to May and then soon the rainy season starts. However, it's not the case always because of global warming weather in Thailand is becoming very unpredictable. Rainy season in the central region goes until October. It gets very humid during this time.
The coldest month for Thais are December and January when the temperatures can go down to 20 degree Celsius in the central region and Thais will start wearing their winter clothes as they're fond of jackets. After the end January the climate starts getting hotter and more drier. The hottest months are April and May with temperatures rising up to 40 degree Celsius. Also, with the rainy season approaching it starts getting humid in the month of May.
The South of Thailand lies in the Andaman and has a cool climate throughout the year. The south of Thailand receives the maximum amount of rain in Thailand. It has just two seasons dry and wet. It gets wet during the months of May to October and dry from November to May.
The temperature is cool from November to February when the northeast monsoon blows cool dry air.