By "coolest" I mean "badass", "awesome", etc.
For me, it'd be the Belgian Shepherd. They're the kind of dog you'd associate with a haunted house - something that would give you goosebumps if you saw it sitting in a beam of moonlight in a cemetery, or silently running across a misty field in the middle of nowhere.
They just look so cool, like how I picture hellhounds. They have that wolf-like look to them, but with pitch black fur. And their fur is so shaggy, except around their feet, which makes them seem like they could be nimble and quick, too.
So...I'm not expecting a whole lot of replies to this, because I'm not sure there IS a more badass-looking dog than a Belgian shepherd.
It just can't be beat.