Superman: The Movie (the first Christopher Reeves) He builds the fortress of solitude and then the giant floating head of Marlon Brando appears and talks and then says, do you have any questions.
Who am I?
You definitely couldn’t show ALL of Angel Heart or ***Total Recall ***or even Blade Runner to kids.
But you could show excerpts dealing with the key issues certainly.
Who am I? I am a man who likes to drink alcohol off the toes of vampire strippers!
Derek Zoolander to his reflection in a puddle:
Derek: “Who am I?”
Reflection shrugging: “I don’t know.”
Not sure if it’s what you’re looking for, as there’s really no epiphany or monlogue scene, but the entire movie Regarding Henry is an exploration of Henry figuring out who he is.
Harrison Ford stars as a douchebag attorney who is shot in the head and suffers amnesia (amongst obvious physical ailments) as a result. His recovery includes learning what a jerk he’d been and resolving not to be that person any more. There’s a conversation between Henry and his physical therapist near the end of the film where they discuss Henry’s choices for who he wants to be.
In Juno, in the scene where Juno tells her parents she’s pregnant, the dad says something like “We didn’t think you were that kind of girl.” She replies, “I don’t know what kind of girl I am.” I thought that was one of the best bits of dialogue in the movie.
Thanks, everyone, this has been pretty helpful actually. Now to put some clips together and figure out a way to transition from the Muppets to Tropic Thunder. 
Conceded. Would require clips from multiple eps, though.
It’s not a movie, but there’s the awesome Act I finale of the Broadway musical La Cage Aux Folles:
Context: Albin’s lover, Georges, has just told him that he’ll have to make himself scarce when the conservative parents of Georges’ son’s fiancée visit. (This after Albin has more or less been the son’s “mother” all his life–Georges’ only indiscretion during his long relationship with Albin was with a woman who had no interest in raising a child.) Albin says that he understands, but when he goes out onstage for his drag show number he breaks down onstage. When the other performers have cleared off, Albin sings this and it’s AWESOME. At the end, when Georges has come out to see what’s going on, Albin hurls his wig in Georges’ face and storms off. Curtain for Act I.
(This is George Hearn, the original Albin, singing it for a concert. I’m sorry I couldn’t find an upload of the original cast album.)