Chrono Crusade

Chrono Crusade has possibly the best ending song of all time. You should watch this anime just for the ending song.

Chrono Crusade is an anime about demons and the church. The main character is Rosette who is an exorcist working for the church. She made a pact with a fallen demon. A demon who lost the source of his demonic powers. The ultimate badass named Chrono. Chrono is an interesting character in so many ways. For one... his human form is chibi and kawaii~. For the other, despite the fact that doesn't like violence and stuff... when he transforms. He's the ultimate badass. Kinda sick. And he's totally the type of character that I enjoy to watch. Because his demon form is pretty much unrivaled in terms of strength.

Throughout the anime you follow the life of Rosette. There are a couple of overarching plot lines. One is of course the power rangers like occurrence of demons that need to be exercised. The other is character development through reveals of how the characters came to be. Turns out in the realm of demons there were these group of demons who rebelled and stuff. And so you have a group of sinners who are supposedly badass because they escaped the demonic organization.

The one thing that I didn't enjoy at all was how Chrono "gave" up his horn. I mean... WTF. So the horn is the source of a demon's powers. And Chrono being the uber badass demon that he is had no equal in combat. But somehow after leading his friends to safety... he lost his horn. DAFUQ. We should discuss this after you watch the anime.

tl;dr - best anime ending song possibly. This or Rewrite from the original FMA. And an OP demon named Chrono. You should watch this if you like demons / vampires / action and stuff.