Because I have a child in elementary school, my favorite time is at the end of May.
We've only done it once (his first time), two years ago but I have been to WDW in July (with a Brazilian tour group), in December and in February.
with a kid in school, the options are the summer, Spring Break (late March/early April in GA), Thanksgiving week and Christmas Break.
Late May was AWESOME! We didn't get a lot of long lines, it was warm enough to enjoy the pool and the water parks but not too hot, we got to do 90% of what we wanted and we even had a chance to come back to the hotel, relax and go back out.
We got there on a Sunday and started hitting the parks on Memorial Day.
Next year we will either go Thanksgiving Week due to how close it is to my son's birthday or late May again.