Singapore (In Asia)11.8 million visitors

   (Just make sure you heed the local laws—something like spitting in the street might merely be considered rude in your hometown, but here, it carries a severe penalty.) You'll find historic sites like the Thian Hock Keng temple, superlative shopping (including gargantuan malls) and numerous beaches. Singapore is known as a bustling metropolis that also happens to be one of the cleanest and safest cities of its size in the world.

                                                                                    

Istanbul (Turkey)11.6 million visitors

   You'll spot Greek, Roman, Byzantine and Ottoman architecture around town. The city of Istanbul is a vibrant mix of many cultures, due in part to its location (straddling Europe and Asia across the Bosphorus Strait) and, well, in part to its history of getting conquered. The Hagia Sophia, for example, was a church, then a mosque and is now a museum.