The best time to visit Disney World and why
No matter how things change in terms of the parks and number of resorts, historically, late January through February has been the best time to visit Walt Disney World. The holiday crowds have thinned out and, excluding holiday weekends, it's typically less busy.
The weather itself is helpful as it's far cooler. It's not freezing by any means, but it can be fun being able to walk around the parks in jeans and a sweatshirt rather than sweating it out.
There's also a chance Disney offers discount deals in that time period as well. Granted, that means less time at the pools, but it's balanced out by how much more can be done in the parks with smaller crowds (which means being able to save on Genie and Lightning Lane).
Another good time to visit is mid-September, after the summer season. It can still be hot with a chance of storms or even a hurricane so be careful and get travel insurance if needed. Early November, right after Halloween and before Thanksgiving, can also be lower in crowds.
Overall, if one wants the best Disney World experience, January through February is the most likely time to catch low crowds and better weather to enjoy Walt Disney World well.