As a teaser, Maré is the wildest, most desolate (and thus the most charming) of the three. Ouvéa is truly a paradise (the photo above was taken in Ouvéa). And Lifou is unmatched for its tropical fish and coral. I thought "Grande Terre" (the main island of New Caledonia) was stunning. The Loyalty Islands win, hands down.
Photo by Laurent Guiader, 2006.