Fall for me.
I just can't stand being to friggin hot. I don't really like shutting up the house and only breathing air conditioned air either. So summer's out.
Spring: There are some nice days in there, but usually too damn rainy and muddy to enjoy much of it.
Winter: Well I like winter time. If I didn't have to drive in the snow, ice, and sleet, I'd like it a hell of a lot more. So it runs a close second to fall.
Fall, you can open the windows and still not burn up. The colors make things more interesting. Not to mention the pleasant aroma that the dying leaves give off. Toward the end you can smell the snow on the cool air, but you still don't have to drive in it yet.
Of course like patty mentioned, there's also football. How can you not love the start of football season?