Sopa de tortilla (sopa azteca), pozole, and menudo are probably the three most popular Mexican soups. With that said, there are so many different soups in Mexican cuisine, it can be hard to determine what is truly the most popular.
It can also from region to region. In coastal regions, rich seafood soups are extremely popular including Siete Mares (7 Seas Soup that includes 7 types of seafood) or Caldo de Camaron (shrimp soup.) Inland areas tend to favor heartier soups like Caldo de Res or Caldo de Pollo.