October and November are the prime months: the monsoon has cleared, the air is at its cleanest, and the Himalayan views from the ridge are as sharp as they get. March and April are also excellent — rhododendrons are in bloom on the Namobuddha trail and visibility is still reasonable before the pre-monsoon haze builds. December through February can be cold at night but clear, and the snow-capped peaks look dramatic in winter light.
Avoid the monsoon months of June through September if mountain views are the goal — the peaks disappear behind cloud for weeks at a time, and the Namobuddha trail becomes slippery. That said, the valley turns intensely green and the lodges are nearly empty, which has its own appeal.