About SE Asia: I loved biking in northern Thailand and northern Laos, but those trips were in January & February, which is by far the best time to visit the northern parts of those countries (and presumably northern Burma, as well), because from December to February, temperatures are much more comfortable than the other 9 months of the years. The north is far enough from the equator that it cools down a bit for 3 months. Further south, it's very hot year-round.

I once biked in Malaysia in March, which was OK weather-wise because it's between 2 different monsoon patterns. However, I think the biking is better in Thailand and Laos.