I'm in Kentucky, so its a long way to a beach. My favorite as a kid was always Clearwater Beach, where my grandparents lived. Then later I discovered the Florida Panhandle, anywhere along there is nice. Atlantic Beach, NC was also pretty.
But this summer I found my new favorites. Sandy Beach, Sunset Beach, and a park with a beautiful beach, I think it was Malaekahana. We only got to O'ahu, but I'd love to see Maui.