A lot of people say the first 2 weeks of December. I say any time that's not the week between Christmas and NY. While some people might like the (almost) no crowds the first 2 weeks, what I don't like about it is the short hours and limited entertainment options.
The third week (at the beginning), the week before Christmas, is ideal, though slightly more crowded. Normal peak hours, 8-12 or 10-10, most rides open (some rides are down in the first couple weeks of December), entertainment going in full swing (2 Fantasmics, fireworks, 2 parades, electric parade every day), and still not very large crowds until the end of the week. We went during this time last year and squeezed in 60+ rides in 2 days + fireworks and Fantasmic and breaks. See my TR for details.