It depends what you
came for
If you want the Himalaya — the real thing, sharp against a blue sky — come in late autumn. October and November have the clearest air of the year, and it is not close.
If you want the valleys largely to yourself, come in winter or during the summer monsoon. You will trade some views for monasteries you can stand in alone.
If you want colour — rhododendron on the passes and the biggest festival in the west — come in spring.
Most people come in autumn. It is the right answer often enough that it has become the default, but it is not the right answer for everyone, and we will say so if it isn’t yours.