Big Island <3. x1,000,000.

Kauai is beautiful too. Maui has it's charm.

Oahu has some real beauty too. Sure, stay out of Waikiki unless you like hookers and malls, but even there, the beach is wonderful. Well, the waves. If you surf, it's a long, gentle wave that you can ride forever and ever. And if you don't surf, you can see happy people, riding that long, gentle wave, and maybe for $10 or $20 you can get a lesson, try it yourself and fall in love. While you see Diamondhead to your right. And sunset in Kailua. And the North Shore, with its big waves (sometimes) and the yummy shrimp trucks (don't think just eat) and shave ice. Or hike to a waterfall or two (ask a local or google). And the drive over the mountains, alternating mist and sunlight, and looking out onto the basin on the east...all better than anything you'll find on the mainland. If you got a great deal on a place to stay on Oahu, then go there! And find the cool stuff, there's plenty. </defenseofoahu>

But, really, go to the Big Island.