Iglesia San Luis de los Franceses, Sevilla
This is a church on San Luis street in the Macarena district. I don't think it's in use as a church any more, but it's still visited. It is a beautiful example of baroque art/architecture and even though it's small, it's absolutely breathtaking. Also, the times I visited, there was a guy by the altar playing new-age music from a laptop, and it gave the church a mysterious/eerie atmosphere.
Often skipped by visitors, this is one of my favorite churches. More like a museum as there are no longer any services held in the church, it is well worth the 20 minute walk from the center.