Ive seen Rec (the Spanish version of Quarantine), Ive seen the Ring (Jap and American) and the Grudge (Jap and American) and thought the American Grudge was still scarier than the Jap version as the storyline worked fluidly, whereas I found the Jap version jump from person to person as small stories.
I love The Devils Rejects but its definately not a classic horror. The Descent is probably one of my favourite, as its a true low budget UK horror flick. Just enough darkness, simple story line and lots of jumpy bits.
Creep is great until you see the twisted bloke who is doing it all, and then it goes to pot.
Let the right one in is definately on the watch list for me, as its had great reviews from mates.
In terms of a dark dark movie, I love Pathology too, and can wholly recommend it as a get under your skin/it could happen to you scenario based film.
For me, I reckon a good horror wouldnt show the evil thing in full until the very very last person dies (or dosent) I dont think Horror’s nowadays are subtle enough to be scary, too much in your face stuff that dosent get you into the film as much as a clever storyline.