I've been to O'ahu multiple times (I have family over in Kailua), Maui briefly and most recently to Kaua'i. I would echo others in this thread in saying Kaua'i is my favorite - it's the most picturesque and also seemed to have the most laid-back vibe.
That's not to say there aren't many cool things to see on O'ahu, but if I had to pick among the three I've been to it would be Kaua'i, hands-down.
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