October to March is the best time to visit Bandarban. The monsoon has ended, the roads are passable, the viewpoints are clear, and the rivers are down to manageable levels for crossing. December and January can be cool in the hills — genuinely cold at night above 2000 feet, so bring layers.
Avoid June through September. The monsoon makes dirt roads impassable and can cause landslides; the hill tracts are notorious for dangerous conditions in heavy rain. April and May are hot and humid but can work if your main interest is the Golden Temple and the waterfall rather than trekking.