When he intellectually pwns the snotty Harvard student who made fun of his friend, gets the phone number of the girl said student was trying to impress, and delivers the best Take That when he sees the snotty student later that night.
"Do you like apples? Well, I got her number! How do ya like them apples?"
When Will shows how an abusive dad simply couldn't better him:
Will: "My father was an alcoholic... He used to just put a belt, a stick, and a wrench on the kitchen table and say, "Choose."..." Sean: "... Well, I gotta go with the belt there." Will: "I used to go with the wrench." Sean: "Why?" Will: "Cause fuck him, that's why."
Let's not forget Sean's "If you ever disrespect my wife again, I will fucking end you" to Will during their first meeting. And later his "The Reason You Suck" Speech to Will at the park.
That speech is easily one of, if not THE main reason why Robin Williams won the Oscar, but Matt Damon deserves some props as well; watch Will's face as Sean lets him have it - he slowly goes from "My shit don't stink," to someone who has been completely and utterly PWNED.
Chuckie blasting Will, "You don't owe it to yourself, you owe it to me. Because tomorrow I'm going to wake up, and I'll be fifty, and I'll still be doing this shit. That's all right, that's fine. I mean, you're sitting on a winning lottery ticket but you're too much of a pussy to cash it in and that's bullshit. Because I'd do anything to have what you got, and so would any of these fucking guys. It'd be an insult to us if you're still here in twenty years. Hanging around here is a fucking waste of your time."
Which leads to the end, when Chuckie finds that Will has indeed left; while he will miss his friend, his frown turns to a grin as he realizes that Will has cashed that ticket.