The best time to visit Thailand depends on what you want to do there! Generally speaking, the climate is hot but depending on which areas you want to visit, the seasons can differ quite a bit. November to March is considered high season when the weather is cooler and less rain occurs. July to November is the monsoon season which brings a lot of rain and humid days. Shoulder months are a great time to visit that will see you miss the crowds and save money on accommodation. 

Thailand climate:

Low Season: July - October (monsoon season)

Shoulder Season: April - June & September - October (hot and dry)

High Season: November - March (cool and dry)