Church of Santa Maria del Popolo, Rome
This small church in Rome lays claim to probably the most immediately impressive of Caravaggio's religious paintings (the Crucifixion of Saint Peter) which is still in it's original space on a side-wall in the Cerasi Chapel there. Many of his paintings were religious so calling this one his most impressive is obviously a subjective consideration; but it's definitely striking.
Like many of the churches in Rome which boast Caravaggio paintings, you pay 1 euro to turn the lights on for 1-3 minutes, so you can admire it or photograph it. It's at a rather extreme angle and you can't walk into the niche to see it from straight on. I've had to use Photoshop to make my photos readable.
The church was also home to the 'Cult Of Death' around the time of the plague and is consequently decorated with all sorts of creepy, little touches...
This is one of the most memorable churches I've ever seen.