Gran Pirámide was the hub of Olmec, Toltec, and Aztec religious centers and is, by volume, the largest pyramid in the world. It consists of seven superimposed structures connected by tunnels and stairways. Ignacio Márquina, the architect in charge of the initial explorations in 1931, decided to excavate two tunnels partly to prove that el cerrito (the little hill), as many still call it, was an archaeological trove. When seeing the Zona Arqueológica, you'll walk through these tunnels to a vast 43-acre temple complex that was dedicated to the god Quetzalcóatl.
On top of the pyramid stands the Spanish chapel Nuestra Señora de los Remedios (Our Lady of the Remedies). Almost brought down by a quake in 1999, it has been wonderfully restored. From the top of the pyramid you'll have a clear view of other nearby churches, color-coded by period: oxidized red was used in the 16th century, yellow in the 17th and 18th centuries, and pastel colors in the 19th century. You can obtain an English-language guide for about $15.
Calz. San Andrés at Calle 2 Norte, Cholula, 72600, Mexico