I've stayed at all three, and Poly is definitely my favorite. WL is a close second after our first stay there this past month. Both resorts having absolutely gorgeous lobbies and grounds, though obviously each with their own unique theme. They also both give you the boat option to MK which is a quick and pleasant ride to and from. Poly's rooms are a bit larger than WL, but that didn't really matter to us during our stay. Roaring Fork, Whispering Canyon, and Artist Point are all very good restaurants in their respective categories, as are Ohana and Kona at Poly. The big advantage with Poly might be that you have the boat, and the monorail to Epcot. We stayed at YC in December and thought the renovated rooms were really nice, and of course access to storm along bay. Beyond that, it felt pretty "basic" to us in comparison to WL and Poly.