A full-month recommendation isn't really possible because each week is different. January isn't busy, but first week is crazy. May is really low, but has pockets of grad nite that are insane. December is manageable or low for first 2 weeks, then insane last 2.
The best bets overall (based on wait time numbers) are January (Low crowds, great weather, but refurbishments), May (Moderate crowds, good weather, low refurbs, but grad nite can be rough), and September after Labor Day. (Moderate crowds, decent weather, low refurbs, Halloween decor)
The tough part is the parks are "always busy". You'll get pockets of less busy, but overall lots of ppl.
Also, during the school year, weekends are better than weekdays on quality and wait times because so many annual passes are blocked out. During the summer the opposite is true. So don't plan your trip to miss the weekend if it's during school months.