The State of Jalisco is a major retirement magnet for Americans. Encompassing Guadalajara, Ajijic and Puerto Vallarta, it is one of the most beautiful regions in central Mexico. Lake Chapala/Ajijic is a town with many foreign residents, but in particular lots of Canadians call it home. The Baja Peninsula also has many American expats due to its close proximity to the US. Almost all of the towns that have US consulates give an indication that there is a sizable presence of expatriates in those areas.
Of course, there are many beautiful areas of Mexico that are untainted by a lot of US and Canadian expatriates. Many prefer this to more touristy areas since they are looking to live in an unspoiled Mexican city or town. The result of many expatriates in one area can be higher prices on goods, housing, and services. So, depending on your preference, the choices are many.