Well, that question is a tough one!!
If you mean by that which is your favorite, I would have to say so far mine is MK. It is also the one I want to see first -and last each trip.
But the one I tend to go back to again and again is Epcot. There is just so much to "do" there that you'd really be hard pressed to do every single thing there (and how could you? eat at every restaurant and see every show too? No way... impossible. LOL) in one trip/day.
It also depends on my mood. I'm sure it's the same for others??
If I'm in the mood for some of that disney magic- MK.
If I'm in the mood for food, shows, or just relaxed entertainment with not a lot of rides to rush to/etc. - Epcot
If I'm in the mood for thrill rides- MGM with ToT and RnR
If I'm in the mood for animals? duh AK.
LOL