True Romance is the ultimate movie for scene-stealing bit parts, but several of the ones mentioned don’t qualify.
Gary Oldman is close, but he was also in the hotel scene with Samuel L. Jackson: “And you’re you…BANG!”
Brad Pitt was in several scenes:
“And…some cleaning products…”
“Condescend t’me, man, I’ll fucking kill ya…”
“Would you guys like a hit? Okay, you go, and you keep going, and then you take a left…”
Christopher Walken counts, but only because his elevator scene was left on the cutting room floor.
I’d go with Christopher Walken in Pulp Fiction, myself. His transition from respectful soldier to “He’d be damned if he was gonna let some slope put his greasy yellow hands on his son’s birthright…” was a thing of beauty.
But for the Kevin Smith fans, Jay & Silent Bob’s scene in Chasing Amy was pretty good.
Technically, this doesn’t qualify because the character appears in just one brief later scene and has only one line.
Orson Welles was brilliant as Cardinal Wolsey in his scene with Thomas More in A Man For All Seasons. Of course, Bolt’s dialogue was also brilliant, so that was a major factor.
Anthony Hopkins was in two short expository scenes of Mission Impossible II; I’d give him the nod. Right before his first scene with Tom Cruise there’s a shot of him watching a parade from his hotel room and the look on his face says volumes about the horrors he’s seen in his day. And then immediately after he banters and spars good-naturedly with Cruise.