May- we felt was the best time of year that we visited WDW; the weather was great, park opening times good and less crowds.
However, give us any month and we'll be there- we love DIS sooooo much.
I don't believe I've seen any March votes....March, that is, when Easter is mid-April. Weather is warmer, crowds are not low, but not overly crowded, and you can see SPRING TRAINING BASEBALL !!! I saw my Yankees last year at Crackerjack Stadium at WDW, and the DW and I are doing he same thing this coming March. Warm enough to swim, while it is still snowy in my home of Upstate N.Y. March is a great time at WDW...though anytime is great at the happiest place on Earth !!!
We are from the UK and can only go in August as the school holidays and my wife is a teacher so it gets my vote - from the middle of the month the parks are quiet, and we also come back home with a nice tan