I don't like torture-porn sicko stuff like Hostel/Saw etc, where it's all about how much blood and gross sadistic acts you can cram in, but I love a genuinely scary movie.
I think The Descent has been the scariest I've seen.
The best, most realistic monster movie IMO was The Host.
I also like Dog Soldiers, The Ring, and Let The Right One In (the Swedish sub-titled one. American re-make was ok too). Paranormal activity (all of them) was pretty scary with lots of jumps, I think I hid behind my fingers A LOT.