SAFD: this seems slightly unfair and a bit boasty, but... I'm semi-local, a 2-hour drive from WDW, so I really do visit at all times of year and I genuinely do not have a favorite. Different times have different things going for them, and my visits are different at different times because of that.
Summer... I LOVE July 4th and really, really LOVE being at WDW for the holiday and days around it! But it's freakishly hot and crowded, so I love how those visits are really laid back with lots of sleeping late, time at the pool, evenings and late nights in the parks, moving slow and not worrying about getting much "done."
December holiday season... too cold for pools, but I spend more daytime hours in the parks. Decorations!!! Festive music!!! More attractions, less meandering. I can usually get my son to join me (he won't visit when it's hot) and that's always special.
Star Wars Day/s... May 4th and 5th. Not the same as SW Weekends were, BUT now we have SW:GE! I was at GE for its opening day, and visited last week for its first SW Day and it did NOT disappoint! Seeing so many Guests in costume, and nearly all Guests in some type of SW garb is just so cool. A full 10 hours of total geek-out.
Late Aug/early Sept... IF I can avoid a hurricane. These often wind up being spontaneous day trips for that reason. Still hot as blazes, but no crowds!
Fall... F&WF has lost some of its luster as the years have gone by, but I still enjoy checking it out. MK Halloween decorations, and, once upon a time,
MNSSHP. High chance of seeing out-of-town friends. Sometimes lower temps, but not always; some fall visits feel like fall, and others are a lot like summer. Good season for spontaneous day trips.
Late January... 100% weather dependent. I don't do frigid temps if I can avoid it, and Jan. is dicey that way. Mostly day trips to enjoy low crowd levels.
Spring... eh. If there's a last place position, this is it. I used to love the F&GF when it was more about gardening and sustainability and less about food booths. The pollen counts are so bad in spring, and my allergies to them so bad, it's a really tough time for me to be outside for long. I avoid Spring Break times like the plague, but spring overall is pretty busy, so less than ideal for me.