Hot weather. During the cold season, my immune system goes on vacation and thus leaves me to catch every contagious disease ever discovered. :/
At least when it's hot you can go out and get some ice cream, or go to the beach, or have fun. In winter it's all dreary and icy and you just want to stay home where it's cold 70% of the time anyway. Besides, summer = vacation. Of course, too much heat is just as bad, which is why my favorite season is spring. The temperatures are perfect for going out anywhere (50-70F), the relief of winter finally being over, and the blooming flowers leave the most delicious aroma in the air (and pollen, but spring colds are preferable over winter colds). :]