Well, I guess it depends on the child. At any of the parks, your little one can ride with you on any ride that doesn't have a height restriction - so there are only a few rides in each park they can't do.
My little guy is 28 months, and he's going for his fourth time in December. He loves Magic Kingdom, but I can honestly say that Animal Kingdom seems to be his favorite park. He loves the safari and the gorilla and tiger walking trails. Festival of the Lion King is hands down his favorite thing in all of Disney World. He really gets into the African music - it always makes him dance and clap. Flights of Wonder and the Pocahontas animal show are really high on his list too. (He LOVES birds and animals of all kind.) He thinks the Boneyard playground is amazing and he loves exploring at Conservation Station. We could spend hours just wandering around the oasis and looking at everything. Plus, he loves It's Tough To Be A Bug. That scares some little ones, though. And, overall, we find a day at Animal Kingdom to be a much more relaxed, laidback, day. We can see everything and have plenty of time to just wander and explore without rushing from place to place and without much waiting in line - all good things with a toddler! We like to end the day with dinner at the Rainforest Cafe, which he thinks is just the most fun ever!
Anyway, just wanted to add a different perspective.