I have been to Oahu, Maui, and Kauai and like Kauai the best by far. It is quite simply the most beautiful place on earth imo and far less crowded than the other islands.
Oahu is highly commercialized and other than going to see the historical sites (such as the Arizona) it isn't noteworthy. The Dole Plantation is kind of neat and The North Shore is nice. Waikiki though (the famous beach next to Honolulu) is about the most crowded beach area on the planet though. Great if you are into people watching and highly developed tourist areas but that is about it. Oahu is the least majestic of the major islands.
Maui is very nice. It has a very California kind of feel to it as most of the folks there are from Cali. Lots of hippies and that kind of thing. Lahaina is a neat little town with some great restuarants. Kapalua is the best golf course I have ever played at. The trip up to the summit of the volcano is fun with the ride down from 10k feet straight down for 26 miles to sea level. The road to Hana is beautiful but very crowded and slow (most of Kauai looks like it). The Hana side is neat, kind of quaint little town. One thing I would reccomend is driving to the "other side" of the island on the way back from Hana. You go from a spot where it rains over 300 inches per year to less than 10 in about 2 miles. The "other side" is totally barren and serene where you can see dry landscapes that go into the ocean and you will be lucky to see 10 people on a 20 mile drive. Maui is a huge island compared to Kauai or Oahu because it is essentially 2 volcanoes instead of 1. It is less commercial than Oahu but far more commercial than Kauai.
Kauai is my favorite. It has the famous cliffs. It has the "Grand Canyon of the Pacific". It has the wettest spot on earth in the crater of the volcano that feeds hundreds of waterfalls. The North Shore of the island up by Hanalei is absolutely amazing. There are no buildings over 2 stories on the island and only about 40k people, enough to be able to do just about anything you could want but not commercialized. Last time we stayed at a little rental on the beach overlooking a small bay with only about 6 other houses and it was very reasonable. Here is the website:
www.rfreedom.comHaven't been to the Big Island but have heard the volcano is the only thing to really see anyway. I would reccomend flying in for a day trip if that is what you want to do.