Inception
Impressive, original story, astounding effects, and superb acting; but, if you don't have a slight bent towards sci-fiction / fantasy, Inception will be harder to get into. Like the story itself, the movie picks up steam as it goes on.
For me, Inception will remain a movie to watch again in the future, but it will not quite ascend to the ultimate echelon of movies. While thought provoking and reality-bending, it isn't challenging. It is a story but one that avoids trying to impact and test our beliefs. Its biggest strength is as a result its greatest weakness - that it is just a story. That makes it more accessible for audiences and still iconic but not in the way that Start Wars or The Matrix are iconic.