DaCostaBR: They Live. I heard a certain recent game had ripped off its plot and I decided to check it out.
It's really good. The whole premise is fascinating, it's funny (especially that fight scene halfway through) and the acting does a good job too.
I always heard about John Carpenter but never watched much of his stuff. Only Vampires when I was a kid and didn't care for it, then a couple of months back I watched In the Mouth of Madness which, despite not really being scary, was a very good and interesting movie. After that one and now They Live I've been planning to watch the rest of his movies.
Both Halloween and The Thing are good if you like horror films. I remember The Fog being moderately entertaining too.