Here's an interesting study by scientists at Cornell University who looked through millions of photos from Flickr and determined the top 10 most-photographed cities/landmarks on Earth.
I've actually been to all 10 cities (if you count me being at Dulles International Airport as a trip to Washington, D.C.) and shot eight of the 10 landmark photos most often taken by tourists, according to this study.
See how you guys measure up. Here's the link to the article.
(Unfortunately, no Canadian cities made the cut.)