Early February and probably late September will be your best bets. I was just at the parks on Friday and Universal was a ghost town. IOA was more crowded but nothing like the summer months, with the heaviest crowd in WWOHP. Aside from the Hippogriff coaster having an unusually high wait (longer than Forbidden Journey) the crowds were tolerable and I was able to get into the wand shop for the first time since the place opened.