If you had time to visit only one island in Croatia which would it be?

I would skip the islands and go to Plitvica national park - you get to walk all day among little lakes and waterfalls. The park is made up of something like 16 lakes, all of them on different level, all connected by little streams and waterfalls.
You can't bathe or swim or do any fishing there - the nature's protected to the maximum. That makes the place even nicer and cleaner and more awesome than you would imagine.
One day is enough to walk all the paths and see all the sights, so you would have the rest of your vacation free to visit any island you like.
More here: http://www.pbase.com/imrola/plitvica_national_park__croatia

We visited Croatia last fall and spent a glorious week on Dugi Otok. We were driving the coast and took a ferry from Zadar. The tourist office in Sali helped us locate an inexpensive rental on the water (although our Australian neighbor told us they found their place on the web). The apartment was very comfortable and the island was very beautiful with lots of places to walk and swim and explore. I agree with the recommendation to visit Plitivica, but the nice thing about visiting an island is the chance to be in a more rural place and really, really slow down and relax.

i realize this question is quite old, but my two cents, after traveling up Croatia’s Dalmation coast for 11 days in 2007, would be to spend loads of time in Korcula.