We have stayed at all 4 of the resorts you mentioned (plus the 4S). I would have to agree w/ rockdoc + chowder. Hapuna is newer than its sister property , the Mauna Kea, and has a much better pool and excercise facility. Both have great beaches w/ the edge in my opinion to the Mauna Kea. Also the Hapuna seems to offer better deals than the other resorts you mentioned. We also like the Mauna Lani as it has a decent man made beach in front and a beach club w/snorkeling about 1/4 mile away. The ML also has a great new spa and nice excercise facility. The Fairmont would be our least favorite of the 4 because of its small beach and that it seems to cater to groups in our opinion. The 4S is in a different league service and price wise but it also has a tiny man made beach. No matter which resort you choose you should have a great trip. Enjoy