There are far too many to choose from.
Here's a list to get you started: http://www.mangago.me/home/mangalist/235645/
hmm http://www.mangago.me/read-manga/shiiku_gakari_rika/mf/v00/c001/ was pretty weird but good. also http://www.mangago.me/read-manga/samenai_yume/ but i like it.
in terms of sex: https://myreadingmanga.info/madobuchiya-nishin-chikuhyou-no-hanayome-orc-bride-eng/ and http://www.mangago.me/read-manga/kanpai_dj_unapai/ (but i dont have a problem with the 2nd one, its hot).
if we r talking about horror, i dont think there is a manga that managed to creep me out :3
Hye people, have you ever read a really fucked up manga ? So hardcore, so strange that at the end you'r like "what I have just read !?!"
Lets make a list ! Give us the titre of the most strange, the most hardcore, the most twisted, the most disgusting manga you've ever read !!!