There are, in my opinion, no bad lagoon views at the Grand. Conch Key and Sago Cay, have lagoon views that overlook the Magic Kingdom.
Boca Chica and Big Pine Key also have views of the MK, but also include the Contemporary, the Polynesian and the Wedding Chapel. All Lagoon views have good views of the Water Pageant.
If you specifically want a Castle view you can request, but it is hard to get, as everyone wants this view. I personally like Boca Chica, as all views are good and it is very centrally located.