I feel that Animal Kingdom is pretty unbeatable in terms of pure aesthetic beauty in Florida. The sheer amount of details that can be found if you're willing to look hard enough is astounding, and Pandora has only helped its case. The way it is designed to be "explored" as opposed to just walked through I think puts it a step above the other choices.
I could see an argument for Epcot as well, though Future World drags the beauty of World Showcase down quite a bit. I could see when (or if, at this point) the vision of Epcot 2.0 comes to fruition it becoming the most beautiful park in the state. As of right now, it seems like a mess of construction walls, not the prettiest sight. Interestingly, with both of these choices, the biggest issue I have with them is their lack of rides, with Animal Kingdom needing more supporting rides and Epcot needing more headlining rides.
Animal Kingdom is gorgeous, but at the same time the whole area around Primeval Whirl has to be taken into consideration and it's truly and eyesore. I'd probably have to say Islands of Adventure, because each area is so carefully crafted and even though some of the garish colours in the comic areas can't compete with some of the lush greenery at Animal Kingdom or Busch, the detail is just astounding in every single crack and crevice of the park.
Animal Kingdom would be a close second though, because most of it is absolutely beautiful. Busch would come third for me.
The turnaround of Hollywood Studios definitely puts it ahead in my mind, if only from an aspect of the amazing work that's been done between Star Wars and Toy Story lands. Animal Kingdom still takes the cake (in turn also with their Avatar expansion), but I'd put Hollywood Studios forward for consideration as "most improved".
I echo Lord Morton, Cypress Gardens was absolutely lovely. However, I can't say the same about it now that Legoland has taken over, as much on the atmosphere has been lost. In fact, Cypress Gardens was the most beautiful park on the East Coast and Gilroy Gardens remains the most beautiful on the West Coast, and all of America for that matter.
I know, I know.. the Disney parks are lovely (Animal Kingdom stands out for me), but a lot of that is man-made and at Cypress and Gilroy it's all nature.
For example, the Tree of Life is amazing, but doesn't compare to this IMO.