I'm with ALWrules about "Heaven on Their Minds" from
Jesus Christ Superstar. You can't ask for a better opening number in a rock musical--it's a real masterpiece of economical, character-driven exposition. It accomplishes in roughly three minutes what it would take two or three scenes in a "book" musical to establish.
Right from the top, here's Judas, he's the central dramatic character, this is his relationship with Jesus, these are the doubts he has about Jesus' approach and they sound like he's an intelligent man with legitimate concerns, and some background on Jesus' growing ministry is thrown in for good measure.
It's not a classic piece of theater by any means, nor does JCS strive to be as a whole, but it's pretty damn good.
"There is no problem so big that it cannot be run away from."
~ Charles M. Schulz