I live in Kailua on Oahu (lived in Calgary and moved here for university... then ended up staying for a year for work). I've been to all of the major islands- my favorites are Maui and the Big Island. I love snorkeling and diving, which is why I like the Big Island so much. Maui is a bit busier though, and there are upsides to that. Molokini crater near Maui is one of the best dives I've ever been on. Plus, right now, both islands are swarming with whales. Whales are not nearly as common off of Oahu and Kauai, as they are off of Maui and Hawaii.
Kauai is a beautiful island and is worth a long visit too, but it has a few downsides: its really small, there are not many nice hotels, and the diving is not very good. The hiking is amazing... I hiked part of the Na Pali coast last July.
I do not like Oahu. There are many awesome spots to visit, but most are not places where a typical tourist would go. Waikiki is a zoo. I can't stand being there for more than an hour. The diving is ok, but the dive shops are terrible for some reason. Compared to the dive shops on Maui/the Big Island, the shops on Oahu suck. If you have any questions about Oahu, let me know. I've been here for 3 years now.
Like many things in Hawaii, the airports are not up to mainland standards