October, for the same reasons as many other people. The weather here is great, it's sunny but usually cool. The trees are changing color, and it's just beautiful. You can pull out your winter sweaters, but still have a nice sunny day to wear them.
I also always associate "Fall" with Oct. and Fall is my favorite season of the year. It reminds me of going back to school, Football games, and it gets me in the mood for the holidays. (Halloween, Thanksgiving and Christmas!)
Lisa