Just watched The Avengers again on DVD with my six-year-old son. We still both agree that all of the best scenes involve the Hulk, such as when punches Thor for no reason, when he thrashes Loki like a ragdoll, and when he "revives" Tony Stark.
Plus, there are so many great one-liners like this gem:
Loki: "We have an army."
Tony Stark: "We have a Hulk."