Ahhhhh yes, this to me is HEAVEN, The Contemporary is wonderful you are very close but the view is at an angle, you have to book a TOWER room, and request MK view, you cant guarantee it there, but weve always gotten it.
The Polynesian, you have to book a lagoon view room, there may be 1 or 2 garden view rooms that you can reach out over the balcony and see them but not so great....And of the lagoon view buildings my personal fave is Tahiti, anything on the 3rd floor facing the lagoon.
And finally the Grand Floridian, this one its actually harder to get the perfect MK view, and Im not sure what you would have to request.
However of the 3 I like the view from Polynesian the best, the fireworks look best from a head on perspective.
Also some people staying at Wilderness lodge have reported seeing the castle and fireworks from there and its a bit cheaper, but it is a little bit further away. Someone else could tell you more about that than me though.
I was OBSESSED with having the room with the MK view and fireworks from the balcony, we did it and it was SOOOOOo worth it to us.