In my experience, as far as park "crowds" go, the three weeks are very similar. We've been in both the first and second weeks of December as well as the first full week of January (after New Year's/Christmas guests leave). Have not actually been on MLK weekend, though, which is probably a bit more crowded than the other times mentioned.
Because of all the holiday activities and decorations, I'd choose to go in early December over January every time! No contest whatsoever as far as I'm concerneed!
Best wishes -