Soldato
I'm not into horror/gore, but I thought I could get away with the film "Cube Zero" as it's only a 15 rating. However, there's a death scene where someone is sprayed with acid and his flesh is seen melting over a few minutes. That gave me a recurring nightmare over the next few months and I was 29 at the time. I found it more gruesome than Elm Street, Dusk 'til Dawn, Starship Troopers etc despite they're all rated 18.