The four boys simultaneously losing their virginity at the climax of the first movie. The details may differ for each (Kevin: a bittersweet ending of a teenage romance, Oz: the beautiful blossoming of a new relationship, Finch: a rampant lust-fueled tryst with Stifler's mom, and Jim: a bizarre S&M encounter with his future love) but it's an fist-pumping accomplishment and well earned for each of the boys.
Stifler gets two in Reunion.
First he tells off his asshole boss in front of the gang and the boss's girlfriend.
Then he punches out Heather's douchebag boyfriend Dr. Ron as he makes fun of Oz and threatens to sue him if he tried hitting him. Stifler can do it because he "ain't worth jack shit."
Actually he gets three. Incensed at some Jerk Ass men stealing bikinis from girls, Stifler goes looking for them. He told each of his friends different stories: he'll flip them off, steal their beer, and use their cooler as a toilet. He does all three, after hooking up the Jerkasses' jet skis to his truck, before driving off and smashing them up.
He also gets one in Wedding after he accidentally kills all the wedding flowers by replacing them at the last minute with the help of the high school football team.