We go to the islands at least once per year, and Maui is our preferred island. Yes, it can be crowded, depending on when you go, and what you choose to do when you're there, but as mentioned elswwhere in this thread, it has better beaches than Hawaii (Big Island) has less overall crowds than O'ahu/Waikiki, yet still has most of the comforts of being in a larage town. COstoc, Target, etc, are all in Kahalui, which is only about 30 miles or so from MOST of the resorts on the west side. The Big Island in neat, but yes, the lava landscape gets kind of old after a while, and the beaches tend to be VERY rocky. The people there are great however, as they are everywhere. Overall, the Hawaiian people are extremely friendly and courteous.
Kauai and Lanai are beautiful islands that are way off the beaten path, with generally fewer "organized activities" than Maui offers, so it really depends on what YOU want on your vacation. Do you just want to lounge around the beach and catch some rays, or do you want to do stuff, like snorkle at Molokini, take a helicopter tour, go to a Luau (Old Lahaina Luao on Maui is best IMO) or see Pearl Harbor and the Arizona Memorial? Try to avoid the multiple island trips, as you lose 1 day for each time you have to travel. If you stay on one island the entire time, but take day trips, that's not so bad, but having to check in/out of hotels, plus the BS at the airports, really wastes that day.