Winter! Which subsequently includes Christmas time :) and outdoor rinks with friends, snowmobiling, hot chocolate, playing in the snow, shovelling driveways, christmas music, cold nights, cold mornings, warming up beside a fireplace, cold breath, frost etc.
For me it is a toss up between spring and fall. I hate being too hot or too cold so summer and winter, even tho they both have the high points just aren't my favorites. With spring you get new growth and the warm weather starting. There is nothing like the first day of being able to open your windows after a long cold winter. Color in the new blooms after many cold gray days is just plain cheerful. Same goes for fall. Things cool down and again you get to shut of the air and open the windows. You get the beautiful colors of the leaves changing. The sun being lower in the sky and the smells of apple cider or pumpkin pie.