While he's made much more accomplished work since, this remains today as the epitome of his entire oeuvre:

Catholicism, guilt, a forbidden relationship, the Mafia, an excellent soundtrack and amazing acting by all involved. It also shows Scorsese's early risk-taking bravura better than any other.

If you didn't like this, do not check out his first film, Who's That Knocking At My Door (1968), a sort of dress rehearsal (and a great one too) for Mean Streets (1973).

Mick