I've only been to Hawaii a few times, but I definitely have a favorite island. Actually it's a place on an island, because the island is The Big Island (Hawaii).
That place is Kailua-Kona, aka Kona. It's the home of the Iron Man competition, some really cheesey souvenier shops, and perhaps the best snorkeling in all of the islands.
We got into this place by accident, really. We signed up for a points-based timeshare (Vacation Internationale, if you care), which gives us time at whatever condos they have in their system (more time off-peak, etc). And their place on Hawaii is on the west coast, just south of Kona.
And also just north of a fantastic snorkeling area. The usual amazingly colorful fishes abound, and we have visited with sea turtles many times.
Kona also is a stopping point for at least one cruise line. The boat shows up on Monday, hangs around for a couple days, and then steams off into the sunset. At some point in my life I should take a cruise..
Most Hawaiian islands have varied micro-climates, and they appeal differently to different folk. Californians tend to favor the east coasts, with their daily rain and lush vegetation. Northwesterners get quite enough rain in their lives, thank you very much, and opt for the west coasts.
If condos aren't your bag, about an hour north of Kona is the Waikaloa Hotel, which is swanky etc - how many hotels do you know have an interior water slide? You'll be visiting the area anyway, because very close to it is Hapuna Beach, a very wide beach with a gentle slope into the sea ... and huge waves. We're talking close-the-beach, occasional fatality huge.
Our routine for Kona is pretty simple. United flys there non-stop from SFO, and you arrive in the early afternoon, with time to load up with the week's food at the Costco on the way to Kona. This is also where I pick up my customary cheap Hawaiian shirt.
After that, it's swim/read/sleep/eat - the condos have a bank of Weber grills, and nearly every meal is a cookout.
Damn, it's only August - I want to go there now.