Used to be Epcot, before they tore down or replaced most of the worthwhile attractions in Future World. Now though, it's still a lovely park with very few things that really interest me (World Showcase is still fun at least).
So Magic Kingdom. As a kid, it was probably the favorite since it had the most attractions that would interest a little 3-9 year old. Later on it would switch to Epcot as i became more interested in just thrills and quirky characters, but now it's back to MK again since Epcot is sort of meh to me. When Fantasyland is done, and smaller-scale retoolings like Tiki Room are fixed, i'm sure i'll adore it even more. Hard to beat the good ol' Magic Kingdom.
For what it's worth, i do have a lot more respect for Animal Kingdom than i did at first after looking past the lack of attractions. It's gorgeously themed with lovely and painstaking attention to detail. If only it had more ride-based attractions and less...ugh..Dinorama...:hurl:
Hollywood Studios is fun, but it's never been a full day park for me. There's a ton of stuff i completely skip and am thoroughly uninterested in. Visually, it depends on the time of day. I like experiencing it at night best. That's when it comes to life. And that's when i visit it. It's probably my least favorite park, despite some very fun and cool attractions (namely, GMR and ToT, not ridden new Star Tours yet).