The cold, by far. When it's super hot your energy is just zapped and you feel awful ;-; It's even worse around here since it's gets so insanely humid, it just adds to the heat.
Of course, the coldest we ever get is like, 30F / -1C, and that's extremely rare. It only drops below freezing a few days out of the year XD; So I guess I don't really know what the extreme cold feels like, I just know that extreme heat (105F / 40C) sucks >.<