Most Greeks believe that Santorini is the most magnificent island of all. Massive numbers of people visit this island every year in order to see the waterlogged caldera, the epicenter of what is commonly thought to have been the largest volcanic eruption ever. If you wish to take in the full splendor of the caldera, you should make a point of arriving by slow ferry.
Santorini is the one island I would recommend to those people who intend to explore Greece. However, if you do visit be prepared for unstoppable numbers of tourists. Many tourist book villas in Santorini for longer stay. If possible, you should visit this island off-season. I advise you to bring warm clothing if you visit during any season but high summer, because there is often a chill wind called a meltemi that is specific to the Cyclades.