Penny's Window
It depends what you want to do.
The same place can be perfect in June and a mistake in July. 155 destinations, scored month by month, for each activity separately — including the months worth staying away, and why. Nothing is averaged into a single number for the year, and nothing is recommended because someone is selling it.

42 of 155 destinations are in a prime month for at least one activity. These are the ten that score highest — on conditions alone, before you say anything about crowds.
Good now, and quiet now. Conditions of 70 or better, at 40 or less on the destination's own crowd scale.
Most places have one or two months that are genuinely better — and a booking moment that comes a lot earlier than the trip. I’ll write when one of those is coming up. No schedule, no filler.
Somewhere missing from the list? Write to info@pennyswindow.com and I’ll score it.