The USA, that's quite evident I'd say. Receives a huge load of traffic from visitors all year around, and it only peaks at Christmas. The most interesting are other countries that lure in vacationers and tourists during Christmas either through being a holiday destination or having a specific culture appropriate to become a tempting country to visit.
I'd say the most visited place ever is going to be NY city's Rockefeller square and the Christmas tree there. For other countries and towns, definitely Vienna in Austria, Budapest in Hungary, Nüremberg in Germany and Zürich in Switzerland, along with Paris and London for the same reason as New York. There's also Vatican city of course with the midnight mass for the believers.