I can't speak much for the older kids, but we just got back from a trip with our 19 month old, and she seemed to like Epcot more than any other park!
The living seas were her absolute FAVORITE. She could watch fish for days

Also, right near there, the jumping water fountains kept her entertained for a while.
She also enjoyed Spaceship Earth (which was nice to ride while Fl had its daily shower)...the stroller was under cover
She was entranced by the big fountain when it was doing it's thing...you know with the music and all that fun stuff (esp at night with it lit up!).
I can't remember what it is called specifically, and we didn't get to go there, but isn't there a very "science center" ish place as well? I'll do some research and get back to you on the name of what I'm talking about. Hands on fun stuff.
ETA: Found it! it's called Image Works-The Kodak "What if" labs - Let your imagination run wild in this incredible interactive playground! Wave your arms and move your body to create magical music, videotape your image and play it back in super slow motion, make up a symphony by jumping on stones, transform yourself into an animal and e-mail the photo to friends, and much more.
Oh! One more thing that she really got a kick out of after dark was the twinkle sidewalks!!
All that, and that is only Future world! She was a bit too young for the whole kidcot stuff, but still enjoyed watching people at the different countries in WS, and especially enjoyed band with the belly dancer in Morocco.
HAVE FUN!