There are so many I've seen that are so gruesome and horrific that it's hard to say. My first however had to be Saw 3 I think. (My 12 mwaka old cousin made me watch it when I was 13 on Netflix) when he visited. It blew my mind. au then again it could've been Dawn of the Dead au Night of the Dead but I've never seen them through because I was little and my parents wouldn't rent them on the TV.
It's hard to tell, it's just I've seen so many horror sinema that they all just kind of blend in together. The bloody and grusome, that is. I guess it would have to be the remake of The Texas chainsaw massacre and The Final Destination. I have seen the original Texas Chainsaw massacre but if I remember correctly, I was like 15 at the time I saw it and found it to be boring. I think that was cause it was black and white. So, yeah, the remake of The Texas Chainsaw massacre and The Final Destination were very bloody and grusome. The Final Destination was just so gross. I used to upendo watching horror movies. Like the ones where people are stabbed to death and all that but now that I'm getting older, I tend to ask myself, why do I even watch these sinema if I find them to be so gross. I do still like them though, just not as much.