My, this is a poser (not to be confused with poseur).
I went with Superman: the movie, because of the character and the actor. Sure, the effects are dated, the script had some clunky moments (Lois' poem, ugh, gods save us). But the way Reeve made each persona so strikingly different, it made me believe that a pair of glasses and some hair gel are a great disguise. No other actor in a superhero role has had me as convinced as Reeve. The only one who comes close is Kevin Conroy.
That said, the X-films, Spider-films, Batman Begins are tied for second, with the Hulk and Hellboy in third.