Many people in China believe that spring and autumn are the two best seasons to visit Hangzhou, specially late March-May and September-mid November
In these seasons, temperature is warm but not hot. Daytime temperature ranges from mid 10s to mid 20s centigrade, and night time temperature ranges from low 10s to low 20s
Spring is the best season to visit the West Lake because all kinds of flowers are blossoming that time. In order: plum blossom, peach blossom, and cherry blossom
Autumn is also very nice and my favorite because of the yellow and red leaves
However, I suggest avoiding the last week of September and the first 2 weeks of October because that's when the city becomes packed with tourists from the golden week holidays