As others have already said, it depends what you want.
Oahu is the most heavily populated island, and the most urban. (Honolulu is a pretty large city; I think it’s still in the top 50 – it’s approx. the size of Oakland, Miami, Minneapolis…) There are still parts of Oahu that are very "country" (the North Shore, Windward side, etc.) so you can "get away" if you want to. The Moana Surfrider was formerly a Sheraton, but converted just a few months ago to a Westin, so that would be an awesome deal for you; they’ve done some renovation work, I think, and I *think* that you can still do the "I’m staying at a Sheraton/Starwood/Westin property so charge that $12 fruity drink to my room there" thing. The "sister" properties are the Royal Hawaiian ("pink hotel"; beautiful, but it’s undergoing renovations later this year, and will be totally closed), Sheraton Waikiki (huge, and pretty much your typical Sheraton) and Sheraton Princess Kaiulani (which is the "budget" of the bunch). The W Hotel someone else mentioned is out of Waikiki, and is also quite nice. Kinda small rooms, tho’, but oh those beds!
The Westin on Maui is quite nice…my friends stayed there when they came down, and they really liked it. Maui’s more relaxed than Oahu, and it’s a nice compromise between having things to do and getting away from it all. If you go, go Upcountry for a day (or try to stay at a B&B there) – it’s beautiful!
Lanai is my fave recommendation for a honeymoon, but you have to be willing to sit and stare at each other (or the beach) for hours w/ nothing else to do. It’s super teeny, and there are only 3 hotels (2 are Four Seasons, one is an independent, and kinda small). You can go around the whole town in about 5-7 minutes. No joke. But it’s gorgeous, total luxe (at the hotels), and sooooo relaxing.
I haven’t stayed in a Big Island hotel in forever, but it’s a great place to visit. Kona is the town w/ the big fancy hotels. You won’t find much in Hilo as far as hotels, but it’s technically considered the bigger town. It’s a cute, country kind of feel. Not sure, but there’s pro’lly a Westin in Kona.
Kauai is also very nice and relaxing. They have a huge fancy Hyatt, but I don’t know about Westin. Again, this is a more "country" island, and it’s good if you like doing outdoorsy things (hiking, kayaking, etc.).