Generally, cool season from November to February is considered to be the best time to visit Thailand. This season is the busiest time of the year. Besides great weather in most parts of the country, you can enjoy events like Loi Krathong and Lopburi Monkey Banquet. Even though you'll have to plan your trip well in advance and tolerate tourist crowds, you will relish seasonal fruits while lolling on the heavenly beach.
March to May period with its lower rates is considered shoulder season, usually deadly hot, especially in the north-east with higher humidity levels. Fun seekers should definitely join water battles during the Songkran celebrations in mid-April.
Southwest monsoon affects northern territories and west coast of the Thai peninsula (Phuket and Krabi areas) from May to October, peaking in September. East coast of the Thai peninsula, facing the Gulf of Thailand (places like Koh Samui), experiences the northeast monsoon roughly between September and December. Just for the record, you still can enjoy your diving and kiteboarding during the low season. Besides, May to October is the best time to get good deals in Thailand.