We've stayed at CR and Poly and from the pix of GF, I'd say that CR is the only resort that offers a "full" view of the Castle, from top to bottom. I was very impressed with that when we stayed in the tower MK view. And while I thought Poly was a lovely view, I was a little disappointed that we only saw the Castle from the middle up. Perhaps because I was on the beach and pool? We did not have an actual room with an MK view. In any event, I do have to agree with CR offers the best views of MK and the Castle, but it could just be a matter of taste or opinion as some people prefer to see just the Castle and not the entire park and other resorts.
To provide the other perspective...
From GF and Poly, the MK view is like a fantasy. For the most part, you see beautiful beach, palm trees, and lagoon. Then, in the distance, you see Cinderella Castle, the peaks of MK's mountains (Space, Splash, Big Thunder), and perhaps some of the other resorts. The view is completely magical.
In addition, I prefer to see the fireworks from GF/Poly compared to CR. Because you're closer to the ground, the fireworks appear to be above the castle, as intended.
From Contemporary, you do see the entire park, but you also see a huge parking lot. Except at night when you can't see as much, the parking lot is very prominent. Also, you can see some backstage areas from your view, which detracts from the magic.
I've never loved seeing the fireworks from CR/BLT. The entire show is lopsided, and most of it appears to be over Space Mountain instead of the Castle. Also, you're basically at level with the fireworks, not really looking up at them.
Some people may not be bothered by the parking lot view, or the lopsided fireworks, because they really enjoy seeing the entire park. But it's definitely a matter of opinion, as people have different criteria for determining a "wow" view.
