My dd 2 has been twice, at 3 months I can't say she had a favorite, at 14 months it was hard to tell, but she LOVED the fountains at Epcot, that was probably her favorite thing anywhere (the ones you can run in)
There was quite a lot she could do at MK, so that was probably her 2nd choice.
Animal Kingdom was good for just walking around and looking at the animals - not sure there were any attractions that really worked for her.
and MGM is not a big hit with either of my kids, they like Playhouse Disney but not much else.
When DD1 was 3 1/2, Epcot was her favorite because there are lots of safe non-scary rides there (she was very very timid on her first trip), she loved tapestry of nations/dreams and Illuminations, the mexico ride, spaceship earth, the land, living seas, figment, loads of things
then Magic Kingdom - quite a few things worried her at MK, she loved Its a Small World, was ok with Pooh Bear and Peter Pan, and even Pirates towards the end of the trip, won't even walk past Snow White's Scary Adventure or the Haunted Mansion. Loves Goofy's Barnstormer (her all time favorite I think)
Also loved the parades and the fireworks.
Animal Kingdom -was happy enough here, but not much in the way of rides for her - she likes Pocahontas, so enjoyed her show, and the safari - we only spent about 1/2 day here that trip.
MGM - again not great for us, but we have to visit for Bear! We love Bear. She is ok in Who wants to be a millionaire, Hated the Little Mermaid 'cause Ursula scared her. Was not happy on the Great Movie Ride - all in all, not a great park for her.