Ohhh, wow, I am really surprised. People mentioned here something I wasn't expecting: Italian horror, although my favourites are from 80's and 90's.
I am a big fan of Lamberto Bava (his father is mentioned here, Mario Bava), Lucio Fulci and specially Dario Argento because my favourite movies from italian horror cinema are Demons movies.
Out of italian horror, I would really recommend Killer Clowns from Outer Space, I.T. (only the first one, but need to watch the second one both to get the gist of the story).
Most of the famous and not so famous gore + horror cinema has been mentioned already. I've watched many many movies from 80's and 90's (I started when I was about 10 years old because my uncle wanted to freak me out and I ended loving them, but not after a few nightmares lol).
Yesterday I saw a movie in the cinema: Until Dawn. Do not watch it, it's terrible, but not only because the argument but because of the actors and script.
Ahh, I forgot, I think was not mentioned here, but Final Destination movies are kind of "it is so bad that it's good" movies :).
If you want some more "contemporary", Slither, it was not an amazing movie but it's alright.
Human Centipede 2
I'm sorry in advance
Don't be, this movie is one big butthead movie :D. I love it and everyone liking horror cinema should watch it.
It's hilarious because the puppets are made by the same person of Killer Clowns from Outer Space movie: Stephen Chiodo. He's a master :)
And by the way, the damn protagonist is the one from Kenan & Kel show I watched when I was a kid. Damn it, you opened a little scar in my heart :3
Let's sing it!