In general, for the northern hemisphere, I prefer to travel in late spring and early fall.
May, and in September or October.
The weather is good; and the crowds are usually gone.
Of course for the southern hemisphere; when its winter in the northern hemisphere its summer there.
So then the best times I believe are October and November for spring; and April for fall.
Visit this website to get a handle on the monsoonal seasons in Southeast Asia.
About Travel: Go Asia; http://goasia.about.com/od/weather/a/Southeast-Asia-Weather.htm;Welcome »