Always has been Magic Kingdom.....in part because it's the first one I remember going to, and also because it's the one with the most attractions that I must do on each trip.
To echo others on here, EPCOT in it's prime was the most amazing thing I'd ever seen. MK is the childhood, all time favorite. For me though, AK is real close. It's about the only place that comes so close to completely transporting me to another world. I could (literally) get lost in the atmosphere of AK. I do agree though, it needs a few more attractions to help round out what they currently have.
I think you are spot on about Animal Kingdom. If they add some more attractions, and keep the detail they have going, it has the potential to be utterly amazing! A new land or two will definately help. They have the space for it. Also a couple more rides the quality and detail of EE and KS will put it over the top. I just wish Kali River Rapids was better. Compared to the Whitewater ride at IOA is pales in comparison, from a track design perspective. The theming is definately better, just not the actual ride itself.