Autumn, not so much because of the colors of the trees (only evergreens where I live... ), but because it's nice and brisk-- not too hot, and not too cold.
spring! the season of rebirth. and bright colors. mild breezes. not enough sun to burn my skin off.
winter is close runner-up because snow is so beautiful and it contains Christmas and New Year's Eve, a very unique sentiment, but the temperatures are just too damn low.