Out of more than 1000 UNESCO designated sites, everyone has their own favorites. Here is a selection from National Geographic, Trip Advisor, AskMen, and Toronto Sun - plus a few of my favorites. According to the Toronto Sun, Petra is the most visited and number one UNESCO site. Bet you can't pick just one to be your favorite!
"A UNESCO World Heritage Site is a place (such as a forest, mountain, lake, island, desert, monument, building, complex, or city) that is listed by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as of special cultural or physical significance. The list is maintained by the International World Heritage Program administered by the UNESCO World Heritage Committee, composed of 21 UNESCO member states which are elected by the General Assembly." (from Wikipedia)