NC-17
You could argue that Ezekiel should just be an R: yeah, it has plenty of violence and destruction, descriptions of stoning and an abandoned baby lying in a pool of blood but that's true for plenty of other prophets' books. But there are certain passages that take it way beyond R. Like this one. You can't really have such a graphic description of bestiality and just settle for a plain old "R"—it's a bit above and beyond the typical "Wrath of God" scenario.