This is a great and terrible question. I say that with all due respect.
It is great because it elicits personal reactions--that is, reactions of particular importance on a personal level. But it is terrible in that movies can have great personal impact in so many different ways.
Films can impact us because they speak to us in some special way, by virtue of being about or addressing come component that is playing out in our own personal lives, or has done so in the past.
They can impact us (favorably) by simply being great films, in any of the ways a film can be deemed to be good. Great cinematography, or acting, or direction can each make for a "great movie" of high impact. The best of them have combinations of great components.
Or a film can have tremendous impact because it simply has a star actor or director that is a favorite of ours, or perhaps is an installment that completes a series, even if that particular film isn't among their best work. The final film in the Star Wars series had impact on me and it wasn't the best of the series, to my mind.
So, even though I think this is not the best way to evaluate movies, I will answer as if you intended option number one: eliciting reactions of a particular personal importance.
The Graduate: Because it related so effectively to my particular stage of life at the time.
Old Yeller: Because it was the first movie to make me cry. I saw it when it first came out. It introduced my sheltered mind to unexpected new adult ways to view the world.
Enemy of the State: Because it was the action film that helped take our minds off of troubles the day I took my two kids to the movies to temporarily escape the fact of their mother's death.