The World's 10 Most Visited Tourist Attractions; Or, 10 Places Not to Go If You Don't Like Crowds:

From Travel & Liesure and Yahoo:

1.) Times Square, New York City
2.) Central Park, New York City
3.) Union Station, Washington, D.C.
4.) Las Vegas Strip
5.) Niagara Falls, New York and Ontario
6.) Grand Central Terminal, New York City
7.) Faneuil Hall Marketplace, Boston
8.) Disney World’s Magic Kingdom, Orlando
9.) Disneyland Park, Anaheim, CA
10.) Grand Bazaar, Istanbul

See the list with more details about each location (and total number of visitors a year) here: http://travel.yahoo.com/p-interests-40711683;_ylt=AhXBUDCGPoeu6MelkIrFj.OhpYMA