For me that would be Autumn.
All those different colours, the rain and fog, the gray sky, the sound dry leaves on the floor make, when you step on them
Autumn feels like the evening of a long, tiring day, when you crawl into bed with a good book and a warm cup of tea.
It's giving you the chance to enjoy the last bits of warmth before the winter comes (and as Ned Stark taught us all there's no escaping it, winter is coming)
My favorite season is autumn, I'm living in a coastal province in China, there is not have so much rain because of moosoon like spring, and the temperature is both not to high and low, feels really good.
Well, perhaps autumn and winter.
I don't really know why :)