Take SF.
Please, somebody!
Most tourists, the #1 biz of the city by far come in Spring Summer and Autumn.
Summer with the all city or West side fog is Always the Coldest.
October and 2 weeks of 'over summer' are always the Warmest.
Winter is by far the Most Uncrowded and with less than 30in/year of normal rainfall and the clearest skies is a good time to "avoid the Crowds".
OK, then, I will say ,,,
Winter and early Spring, before the Crowds arrive.
What about Paris?
