I don't have a ranked list but the scariest I can think of are the Spanish ā¢REC and even more so the 1st sequel ā¢REC 2.
These are in Spanish and not dubbed in English so you must read subtitles, but that only adds to the shaky-cam (much better than in, for example, Blair Witch). Dubbed would probably lose a lot, taking away from the surrealism.
ā¢REC did have an English language remake. That was decent but not up to the scariness of the original. There were also more sequels but quality really dropped off.