I love mid-May (right after Mother's Day but before Memorial Day). The weather is usually pleasant; very few storms and the temps are usually pleasant. Historically, the crowds have been lower, so fewer people in lines, less waiting, easier to get PS to your favorite restaurant -- even same day PS.
Add in the Flower and Garden Show at Epcot and it's wonderful. The F&G Show is something to see -- all the topiaries are really creative. They have lots of demos and tons of flowers of all kinds to see. Highly recommend this time, especially if they like gardens.