Ingmorious basterds for mine. That whole scene is the most tense I've been in a movie theatre. Another already mentioned is scream. Was not expecting drew Barrymore to not make it more than a few minutes.
One not mentioned yet and left field considering the fact most of these are at least good movies with great openings, idiocracy. s**t movie but the opening was both funny and scarily accurate.
Commando has got to be up there. A couple of executions and then straight into a montage of Arnie eating ice cream and feeding deer with his daughter. Instantly hooked.