I'm biased because I worked at Walt Disney World so I am of course going to say Magic Kingdom.
But in terms of what I have been to:
1. Magic Kingdom
2. Disneyland
3. Animal Kingdom
4. Disneyland Park, Paris
5. Epcot
6. California Adventure (This was before the redux)
7. Hollywood Studios
8. Disney Studios, Paris
But by all accounts DisneySeas in Tokyo is the best and it's not even close (I haven't been).