Personally I always, ALWAYS give a movie a black mark when it displays or glorifies animal cruelty. I can watch alot of violence to people, but something in me becomes very angered and upset when it's animals being hurt or tortured.
I'd have to say that for me, the most disturbing was 'Ichi the Killer' or 'Sympathy for Mr Devil'( is that the correct title?), several of the tortures in these movies, but not really for the effects- more for the need for them, the stories themselves. Overall, I didn't enjoy Ichi, but that has more to do with the rambling tale than the overtly disgusting violence and torture.
Also, after reading American Psycho, I'm glad that the movie was as 'tame' as it was, and not more faithful to the book. As sick as the movie was, the book was way, way beyond it.