I completely disagree with DJ Iceman about the Mauna Kea. It is the best hotel on the big island with one of the best beaches in the world. The Marriott Waikoloa pales in comparison to the Mauna Kea...not even worthy of a comparison really. One is a convention type hotel that is on a man-made beach in a touristy shopping mall area. The other is a resort hotel on Hawaii's finest natural beach.
As much as I disagree with DJ about the big island is as right as DJ is about Maui, the Ritz Kapalua is an awesome hotel.
That said, I'd happily spend a whole week at just the Mauna Kea. I wouldn't want to do that at any other Marriott property in Hawaii.