The Thing. The premise is amazing, and it's very well-done, however Kurt Russell's performance as the biggest movie badass is what keeps me going back. He's a damn helicopter pilot surrounded by a bunch of scientists, yet when shit hits the fan he immediately takes charge, telling them nerds what to do. The second they question his command, he threatens to kill them all. It's believable, too. There isn't a moment of the film where I'd call bullshit.
With that said, Alien is probably top 15 all time for me. It's almost as good, but The Thing murders it with replay value.