Steve Buscemi is always amazing. There are way too many awesome roles he's done to just pick one, but I will anyway.
Danny McGrath from Billy Madison. Billy (Adam Sandler) has had his epiphany, and decides to look up the guy that he and his buddies used to pick on back in school, Danny McGrath, and apologize. He does this, and the camera stays on Danny (Buscemi) as he hangs up the phone, gets a WTF look on his face for a second, then shrugs as he decides that the call was sincere. Then he crosses Billy off of a list of people he's going to kill, now that he (Danny) has grown up and become a serial killer.
That alone made it a memorable performance, but then Buscemi's character actually ends up playing an important part in the resolution of the film, showing the importance of maintaining positive karma.