The cool months from November to February are considered the best to visit Myanmar. The temperature is the most favorable for hiking and sightseeing. However, this is also the busiest time when securing accommodation can become problematic.
The only downside of this season is that when travelling by the rivers it can be slow as water levels are low. Mind that lakes and Himalayan foothills are far cooler than the southern lowland and coastal regions, you can even see snow between November and January. Evenings can get quite cool throughout the country sp bring a light jacket.