I'll have to say Berserk too. Its shocking but not just for the sake of being shocking unlike crap like Elfen Leid and other pure gore-fest torture porn rubbish!
Check out GANTZ too. Like Berserk, its shocking in a more refined, tasteful way. Its also pretty damn bizarre, which is another plus for me!
Also the guy above me said Uzumaki/Spiral. This guys on the money!! That manga is weird as hell but thats what I liked about it. I don't think I read much after the "light-house" and the "Snail" chapters. Not because I lost interest, but because StopTazmo took all their files offline! That site just seemed to have almost every manga you could think. Thousands upon thousands... Berserk, GANTZ, Uzumaki, it was all from there!