I really like to travel to Thailand in May. I know that there is a chance of rain and that it is more humid but this is counter balances by it being quieter and there being so many good deals around.
It would depend of course where you are thinking of going because some of the places are more difficult to get to during this down season.
Once you get to July and August it is main holiday season and the prices go up and there are lots of kids around. although you may be travelling with kids?
Christmas and New Year are the same except a lot more expensive.
Jan, Feb and March has good weather also April, but watch in April for songkran as this can be a hard time to be travelling around.
In short any time really dependant on where you are going and what you want to do.

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Kiss
God laughs at people who make plans.