Part of picking a favorite island is because of the resorts where we like to stay.
#1 Kauai for it's natural beauty, fewer tourists, and because we love the Point at Poipu (in spite of the special assessment). The pool has swimming lanes, the pool side bar has delicious mai tais, you can watch sunrises and sunsets over the water, an ocean front room on the corner has a very spacious wrap around lanai, and my favorite part is getting mesmerized by watching the waves crash on the rocks.
#2 Maui for the beautiful beaches, plethora of restaurant choices, sailing charters, and the Ka'anapali Beach Club for its large pool, restaurants within walking distance, whale watching, and ocean front location.
#3 Oahu - We still like spending at least 3-4 nights in Waikiki. The Outrigger Waikiki is the nicest hotel we've ever stayed in. We usually rent a car for a day to drive up the east shore, watch the waves on the north shore, and scurry back to Waikiki before rush hour traffic gets bad.
#4 Big Island - Still haven't seen hot lava flowing or been up Mauna Kea for a star gazing tour so we'll have to go back. Casa De Emdeko is close to all the restaurants and shopping in Kailua-Kona, but a good place to start a drive around the bottom of the island and up to Volcanoes National Park.
#5 Lanai - We only took a day trip on a catamaran out of Lahaina, but would recommend it to anyone staying on Maui.