Honestly, any single week of the year. I don't really care about "crowd levels" or stuff like that. I love being there.
I know most would disagree, but my favorite two weeks of the year to be there are 4th of July and Christmas. Really. Yeah, it's hot in July, but that makes the water parks, Splash, Kali all the more fun. Plus it's just a fun time of year. Xmas is my favorite holiday...so combine it with my favorite place to visit and I'm in heaven. We've been to WDW for the past three consecutive 4th of Julys (and will be again in '18), did Xmas '15 and are doing Xmas this year. I can't wait....we'll be in MK on Xmas morning, and are so excited!!