I've travelled through Europe only (couldn't say "in the world"!). There are several places that are my favorite for now!
I love volcanic / changing places, like Tenerife (driving around the island definitely is an experience), Iceland... And then, I love medieval cities: San Giminiano (Italy) is my favorite. As for big cities, Napoli (Italy) has to be the most amazing one I've seen - living it is really immerging yourself into another culture.
But I think the really best place to be down there is atop of a mountain in the Swiss Alps, trying to spot a marmot, an eagle or a zool :)