I may have disagreed with many of Esquire's "essential" manhood movies, but I heartily second their recommendation of Serpico as any sane person would. I revisited this film a few weeks ago, and it doesn't matter how many times you see it in your lifetime, it never ceases to be absolutely terrifying. It may be a biopic (and perhaps even more frightening because of it), but I think of it as The Turn of the Screw set in 1970s New York thanks to the beginning. Sidney Lumet doesn't tease you with Serpico's fate, he kicks off the film with it, and the entire movie is a long, tense, paranoid ride to get to that gunshot.
Everyone points to Dog Day Afternoon as the fiilm that really flaunts Al Pacino's skills -- and I wouldn't dare argue with that, but I don't think this performance gets enough love. Pacino is another reason I love this scene, as it quickly switches between the battered and bleeding detective, and the young, idealistic rookie who is celebrating graduation with his family ... and all to that melancholy, unconventional theme. It's all in the performance (and mostly the eyes), and not in cheesy age make-up or hammy "Oh, it's tough out here for an undercover cop" handwringing. I don't know if it's an essential man movie, but it's certainly one that should be watched more often.