Seinen series are far more enjoyable to me than a regular old Shonen or Shojo series.
For those that don't know what Seinen is... Seinen manga are series that are aimed at male readers ages 18-30. While a variation on the Shonen style comics, Seinen series contain more graphic violence and adult situations. (Of course the audience also covers a fair amount of girls and women as well as middle-aged men who read Seinen manga.)
The female equivalent to seinen is Josei.
My favorite Seinen series so far includes Gantz, Ergo Proxy, Elfen Lied, and Homunculus.