While I would find it hard to pick out a scariest moment in cinema history, my wife and I do watch a lot of horror movies.
One that I often recommend to people wanting something with some good scares and disturbing scenes is [REC]. I remember we bought the DVD on a whim and found that the menu screen alone freaked us out!
If I had to personally pick a movie that has scared me enough that I will avoid watching it again then it has to be 28 Days Later. At the time I was both living and working in London and the film’s story falls so much into the 'it could happen’ category that it freaked me out completely.