Hard to say really, he's done films which are outstanding for different reasons. JP for groundbreaking SFX, Schindler's list for creating an emotional masterpiece out of a sensitive subject, Jaws for the techniques used to build tension (sound, cinematography, production)....
I've gone for Jaws in the end because it's the closest thing to a horror movie which I actually enjoy. That scene where the camera shot widens while simultaneously zooming in on Roy Schneider is just pure genius from a film making perspective too.