I'm surprised that no one has mentioned Elfen Lied. That would be by far the most violent anime that I have watched. Blood+ and Akira really does not compare to that anime. There is tons of blood and so many heads exploded or ripped off. Lucy goes so far as to use the comedy relief character in the episode as a human shield to take a hail of bullets.

Blood flies and flows like water in the anime, though more so at the begining than near the end. At the end there is a less fodder to be killed. But it is still the bloodiest and most violent that I have seen. I would say that Akira is about the only comparison which has the old tradition of containing tons of blood. It is definitely violent, but it just seems more tame.

I would imagine that Berserk would be very violent from what I hear. I'll have to check it out sometime to see where it stands.