I've been to Hawaii many times. In Honolulu, I stayed at the Hyatt and Hilton hotels which were both very nice. In Kauii, the Prince Hotel is nice but pricey. In Mauii, the best hotel is the Grand Wailaia but also very pricey as well. The nice thing however, about the the Grand Waliaia is that it has a good gym for a hotel gym, with dbs going up to 100 pounds and they have some machines as well.
If you have never been and you like the night life than Maui is the place to go and the Grand Wailea is a full resort and the best place to stay, great golf course, laid back as much as you want.
If you are looking for a place just to kick back and relax on the beach the big Island is fine for that.
Oahu, Big Island and Maui are all nice. If you have the time I would recommend island hopping. If you were to pick one island, then look at Maui or Oahu, though we also like the Big Island, where else can you see an active volcanoe with lava flowing into the ocean? In Maui, anywhere in Ka'anapali is good, and in Oahu, places like the Outrigger Waikiki are in good locations for a good price. If you are going, shop around, there are many specials going on for hotels right now.