Winter. These days, mostly because it's cold, and I can walk very easily in the cold. I like wearing my winter jacket too. Snow's pretty nice until you walk in it. The holidays are also here but I usually spend them working and being alone during the actual days so I don't look as forward to them now as I did when I was a kid.
Autumn is a close second. I love the crispness in the air. Not quite cold enough for me, but it's cool. The leaves are really pretty, too.