I picked May.
I've never tried September due to heat and humidity that is number 1 for least crowded in the poll at the moment.
For January, another popular choice and November, another popular choice, interestingly enough and unexpectedly, I have experienced really heavy crowed in the past two years that I tried these. The one November, many others who had to cancel because of hurricanes seemed to reschedule for our week and we experienced crowd level 8s and 9s per touring plans all week (5s, 6s and 7s were what the prediction was). The one January that I went (2019), early in our week we had light/moderate crowds (4s and 5s), but OMG we were there the weekend of some special NFL event and our day at the MK was a crowd level 10 per touring plans and felt like it too (20 minute waits even for things like People Mover and Carousel of Progress that almost never have waits). Thanks goodness for FP+ and long park hours, and we still enjoyed ourselves. (I was expecting lower crowds than we had both those times, and I'm just commenting that even with well intentioned plans and studying crowd calendars, you just never know.)