Ah, I would have to say that the most shocking manga is Jisatsu Circle, and then the second most shocking manga would have to be Sky High.
My reasons?
First of all, let's talk about Jisatsu Circle, shall we? The number one theme of that manga, if you've ever seen the movie Suicide Club, is... well - suicide. My second reason? It's got tons of pornographic manga images. This manga is definitely not for the faint of heart, either. Activities in this manga include a group of over 50 school kids lining up on a train platform in Shinjuku, their hands linked, and then jumping off right when the train comes in their path. Needless to say, there's tons of blood, but wait a minute! There's one survivor of this horrible tragedy - a girl who would eventually become the leader of this Suicide Club. She also has many anti-social problems, like selling herself. It's a pretty good manga, and I suggest that you all read it.
Next, there's Sky High, and I say it's probably one of the most shocking because it's the second manga that I've ever read that honestly has very mature situations. In this manga, the story is that there is a guardian named Izuko who will guide you in your choice of the afterlife. You only have three of them too. One, you may go back to Earth and haunt one person forever. Two, you may return to Earth for a short time and take revenge upon the one person who made your life a living hell, which basically means that you end up killing that person and then go to hell for all eternity. The third choice is that you may pass through the gate of rage and into a temporary afterlife until your soul in reborn. It already sounds like a very controversial manga, doesn't it? Most of the stories are only mini-stories, but some of them actually do tie in together. The stories deal with the lives of school girls all the way to the elderly and how they've held up until their unjust deaths. There was also a sequel manga created called Sky High Karma, which I haven't really had the time to download yet. The movie is also pretty good, and it stars Yumiko Shaku, who - if you've ever watched The Princess Blade - you know of. I recommend that you read Sky High and see the movie as well...