I think this is tied with Batman Begins as my favourite Batman film. It's an animated film (don't click back just yet) but it's surprisingly quite dark and gritty. It's about this figure called the Red Hood, who is kind of like a less moral bound Batman that likes to decapitates people, and his connection to Batman. It's a bit difficult to describe without spoiling it but it's a great little film.
Here's a trailer although it doesn't really do a good job of conveying the tone of the film.