Man I love a good horror film.
Firstly the Babadook is meant to be one of the best modern horror films, comparable to the Conjuring - I don't know how anyone could deduce from the trailer alone that it looks terrible. I thought it was one of the best trailers I've seen in the past 12 months and I can't wait until I find a decent download of it.
The Thing, The Shining, Alien, The Conjuring, Rosemary's Baby, The Ring (both are good films), Blair Witch, The Brood (Cronenberg), plenty more.
Paranormal Activity 3 is a weird one as me and some mates were high as feck when we went to see it at the cinema, and it was probably the scariest experience of my life. I'm pretty sure if I rewatched it, it wouldn't have the same effect...
My favourite is probably REC. God damn, that final scene in the attic is horrific. Even the 2nd isn't too bad, but I haven't even bothered with the rest, they seem to completely butcher the franchise.