As you might imagine, there are a TON of churches in Mexico. I imagine I'll go to many more, but I've been to three so far.
The first was Santo Domingo in Oaxaca City. It is immense and gorgeous and almost entirely gilded inside. It definitely seemed to be more of a tourist attraction than a commonly-used church. I actually felt a little strange about it; it's weird to see so much wealth when right around it are shacks built out of corrugated metal and crumbling bricks.
The second church I went to was in Zaachila. This church was much more simple, with white-plastered walls and simple wood fixtures. There were mannequins of various Catholic figures in niches along the walls dressed in elaborate and rich clothing. This church was definitely more in use, and it was a comforting place to sit.
The third church was by far my favorite, both for its looks and the story behind it. (The photo above is of this church.) The Ex Convento de Cuilapan was built around the 1500s, and it was so over the top and so over-budget that the bishop actually ordered construction to cease. Although unfinished it is now classified as a national treasure, and draws visitors from all over Mexico and beyond. The stone work is absolutely amazing, and even more incredible; there were hardly any barred doors or velvet ropes. I was able to wander freely around the building and up various stairs and into assorted rooms. Right nearby the church was a school, and it was also cute to see all the high school sweethearts trysting on the basilica grounds! Definitely worth the bus ride out to see it!