One of our favourites, it is in the Dodecanese group of islands and easily accessible from Rhodes. Great for walking, with waymarked nature trails.
Tilos is not obviously beautiful like neighbouring Symi but it has a certain something that makes you keep wanting to return. We stayed there in 2001, 2004 and 2007 and it has changed quite a lot during that time. You can now walk along the waterfront all the way from the harbour to the town beach, thanks to a newly constructed promenade. The main town of Livadia now even has a bypass, so the coast road can be closed in the evenings.