Pooh's Thoughtful Spot at the Magic Kingdom was a hit. This was the playground with the water feature from my "Bravery" post. I loved Pooh's house and seeing Elizabeth play inside it, although there was a little boy who was in there with her and he was in need of better parenting. There were also giant honey pots to crawl through, and fallen "logs" with built in slides, and a surprising amount of room for a little girl to run.
The Boneyard at Animal Kingdom had amazing theming. It is set up to look like a major paleontology dig site. And its HUGE with multiple levels and many slides and rope ladders and bridges and caves and hidden places to get wet. I loved the way they made the climbing area look like scaffolding. It looks totally dangerous, but you would have to work really hard to fall off.
Although it was pretty small and extremely crowded, my favorite playground that we went to was the Honey I Shrunk the Kids playground at Disney Hollywood Studios:
There was a slide made to look like a roll of film, giant mushrooms, a leaky garden hose, a giant water gun, a huge bee, this Tinkertoy:
And of course, a giant ant.
The littlest playground was in the Magic Kingdom across from Mickey's house.
You have to be under 40 inches tall to play there, but it is completely shaded.
We pale people like that.