Maui is our fav. island. It basically has 2 main resort areas - Wailea and Ka'anapali / Lahaina. Wailea is home to some of the more upscale resorts like the Four Seasons, the Fairmont & the Grand Wailea. We stayed down that way once. I personally think there is more to do and better beaches in the Lahaina / Ka'anapali area. There are a lot of nice resorts on Maui - we have stayed at the Marriott & the Sheraton in Ka'anapali and are trying out the Westin Ka'anapali Villas this summer.
On the Big Island the Kona side is the more populated tourist side (vs Hilo) and there are more resorts to choose from. I see both the Marriott and the Hilton Waikoloa mentioned a lot there. (When we went to the Big Island we stayed in an older 2 star hotel in Keauhou Bay that I would not recommend - actually, I don't think it's even still open any more. It was part of the Outrigger chain, which in general is fine, but this one was not so great. We booked it to save money in order to do a helicopter ride around the island with Blue Hawaiian but I wouldn't stay there again if it WAS still there).
We have only done a day trip and have not stayed on Kauai so I can't comment on that island.
We rely a lot on Trip Advisor for our reviews when choosing a new place to stay...