Licia:
"Best" to me means good weather (not too hot), low crowds, and reasonable room rates.
I would go ahead and mark Spring Break off your list because it only meets one of the "best" criteria: weather. The crowds are just too thick for my liking and you'll be paying some of the highest rates of the year if you stay on property. Your May dates are very doable, but keep in mind that Florida weather in May can start to get pretty hot and you'll begin to see afternoon thunder showers. The crowds are certainly lower than your Spring Break dates and you'll pay regular rates.
October happens to be one of my family's favorite times of the year to visit. You'll pay the same rate as you would in May, typically, unless you go over Columbus Day weekend. The weather is pleasant and crowds are similar to May. What makes it our favorite time? Fall decorations are up, adding a twist to our favorite Disney parks. If your family enjoys dressing up, think about including
Mickey's Not-So-Scary-Halloween Party in your vacation. To me, I'd go with October, but if you're worried at all about school, your May dates will provide a magical time, too!
Wishing y'all a magical vacation!
Whitney