From least favorite to most favorite. This is going to sound like I hate WDW, but I don't, I'm just taking the cynical approach the thread and pointing out what I don't like. There should be a counter thread of your top parks and why, so we can highlight the good in all of them. But here's the list from least to most favorite:
Animal Kingdom - So much walking for so little reward. Now, I'll admit, I'm not a big fan of roller coasters, so those don't do much for me here. The dinosaur time travel ride was largely forgettable, which is a shame, since I'm interested in both time travel and dinosaurs. I do enjoy the water rapids ride, but lately I've been going in the winter, so for the most part I don't need to get soaked. I do enjoy the Safari ride and am sort of tempted to go back for that next time since I am now part of the DSLR camera world. While I appreciate what they have going on with the park, it just doesn't do much for me (perhaps because I can go to the Philly zoo anytime it loses some of the wow factor).
Hollywood Studios - Don't get me wrong, I do enjoy going here. However, once evening starts to set in there are less things to do. I'm not quite sure why they don't have many shows going into the later part of the day. Though, the car show is pretty awesome and I don't think I have missed Beauty and the Beast any of the times I have been to WDW. The Toy Story ride is a good time. But there is a lot of dead space. The whole street set in the back is pretty much useless during the day and only useful at night when it is all light up. The back lot tour needs some updating, though it is pretty cool (but really, Pearl Harbor hasn't been a relevant film in forever, and the props they had are getting old, so it'll be cool to see what they do with it, but from what I have watched online, the similar back lot tour in Universal is cooler, and I'm also a little annoyed they stuck Frozen right where the cool walk through of the prop warehouse was). The American Idol thing was dated when it got there and the empty Drew Carey building is an eyesore. Also, if I recall correctly, they used to have a walk through of a working sound stage somewhere in the park. Haha, the glorified trailer walk through for Narnia.
But I actually do enjoy a lot here, the shows during the day are all entertaining, the Frozen show is also pretty fun, as said, the car show is awesome, Toy Story is cool, Fantasmic is great, the movie ride never gets old, and the whole atmosphere there is great, I love that old Hollywood vibe they made.
EPCOT - Love this park, sort of. The countries, great, absolutely fantastic. Food, shows, the shops, all of it is great. Not sure how I feel about their big firework show, but it's still pretty cool. But then there's the other half of EPCOT.
Innoventions is, well, not so innovative these days (I say get big companies like Sony, Google, Apple, etc. to have pavilions set up in there, really get the tech showcase in motion, yeah, it's basically just a giant store then, but who cares, that's where the innovation is). There's the big brown building behind Test Track, that is basically a glorified bathroom at this point. Not sure if events are held there, but it's mainly just an abandoned looking eyesore. And I'm not quite sure why that ride with the purple dragon is still there, talk about lame, and not in the cool, campy, old school/retro type of lame. And I still think the entrance of the park is too far away, but that's just a personal preference. But in all honesty, Spaceship Earth and the countries definitely make it worth going there, so much time can be spent hanging out and relaxing at the countries.
Magic Kingdom - Probably the popular favorite park and for good reason. Nothing quite captures the essence of WDW like MK does. The mainstreet music coupled with the design of the buildings is simply perfect, especially if you happen to walk into the park while the train is up top at the station. Each section really captures what they were going for, with my favorite area aesthetically being (excuse me for not knowing what it is called) where The Liberty Tree Tavern/Harbor House are. And Adventureland is always a favorite area. The only section that is lacking is Tomorrowland. I think they need to play up the retro theme more, there's just a little too much boring, grey walkway and not enough decoration. The noodle station (that's what it is, right?) is closed pretty much all the time and is this strangely empty spot. I also wish the taco place was open more often. Not really a fan of Tom Sawyer's Island, it's just boring and they have Aunt Polly's closed basically all the time. But this is always my go-to park for fun and hanging out. Even though I have been there a lot it just never gets old. It's like a home away from home, being there so many times everything is familiar and I can go at my own pace, not always trying to rush to do everything (though, when it is extra hours the rush is on).