Well, every time is the best time to visit Manali. If you would like to enjoy the snow and the snowfall related activities then do visit in winters.
If you like to get rid of summer heat in plains, visit Manali in summers. Visiting Manali in monsoons is also a good option because Manali's beauty is worth seeing after the rainfall. But there is one drawback of this visit and that is heavy landslides. So do plan to treat carefully and enjoy your days. It is really beautiful place to visit 👍