The Exorcist. Been a few years since I've seen it and I don't know if I have it in me to ever see it again though I'd consider it a great film. Those other horror movies don't scare me unless it's mental/psychological horror films based in reality but this one goes even further for me.
Being a Christian, it was terrifying to see a man of faith lose his faith during that encounter with the demon and kill himself. That part terrified me more so than the demon. That movie alone was enough to get me to read the whole Bible.
The Exorcist is my all-time favorite movie. I was surprised that nobody had it in their Top 40 list from a few months ago.
BTW, Karras didn't simply "Kill Himself" at the end of the movie. He martyed himself and sacrificed his own life to save Regan. According to orthodox demonology, once a demon leaves its host, it may not return to that same host. He provoked the demon (Pazuzu) to enter himself and then threw himself out the window, knowing that committing suicide with the demon inside himself would essentially free Regan and end the Exorcism.