Black Lagoon ... seriously fucked up shit. I have yet to see ... another show I can't recall the name. I'm waiting for it to be done on BD so the gore can be uncensored!
Code Geass doesn't fit in the Darkest Anime group, but it also had some fucked up shit (season 1 specially).
Guilty Crown ... this got fucked so hard it took a large dump on people's dreams. But ... HELL YEA INORI!
Oh ... oh oh oh ! One more... god ... Yes!
WHEN THEY CRY!

Omfg that is TOTALLY fucked up shit with cheese ontop. Omg it was AWESOME! It needed MORE blood and gore!
And finally ... we come to
Elfen Lied. The
first anime to make me think Anime is not what I thought it was ... this is by far the bloodiest UNCUT gore that was burnt pretty hard in my memory. I marathon it at least once every two years.
There's a couple more ...