Gantz is pretty messed up but by the time I came to it I had attained a huge liking to gore.
Same for Corpse Party.
To be honest, the only ''messed up'' scene that I saw that made me want to puke was in Attack on Titan.
I know you said not to mention it but it was the only one that had any effect on me.
It was the scene where Sascha (I think) went to signal an attack from the titans and then you see a small house.
In that small house was an old woman and a 3m class titan, the way the titan was holding her and the perspective that they showed it looked as if the titan was having sexual intercourse with the woman.
Luckily he was just eating her legs, literally.
In the corner there was a young child, paralyzed by the shock, watching his mother/grandmother/guardian being eaten by a titan in front of him/her.
That was the only time that I've really felt sick watching an anime/manga.
Note: If I remember correctly, the scene that I just mentioned has not yet been shown in the anime and is currently manga-only.