What is a good, gruesome anime/manga?

I see someone already suggested Elfen Lied, I'm personally not a fan of it, and from your post I doubt it'll be what you're really looking for. I can see Hellsing being a definite possibility, but my number one choice is without question: "Higurashi no Naku Koro ni" ask anyone who has seen this anime and they'll tell you it's easily one of, if not the greatest anime horror to ever be released. It's violent, it's creepy and it's just plain awesome. Check it out.