Oh come on! How come so few people have said autumn? (or 'fall' for those using American English) As Katiejo911 said, you have the crisp fresh air, the ever-changing leaves. It's a welcome break after a long hot summer.
I'd like to chose winter, if I lived somewhere where they get proper, long winters, somewhere like Canada, Russia and Scandinavia. I love the cold and the snow and ice, but here in England such and winter only lasts for a few days at the most.