Best movie cameo appearance

Ain’t you kids ever seen a spaceship before?

So great on so many levels since those kids saw Loki and his armada.

 

Barry Corbin as Ellis in No Country For Old Men.

 

See also: Dom DeLuise

 

Xavier McDaniel in "Singles": Steve, don't come.

 

Ed O’Neill in Wayne’s World. Although he was in two scenes instead of one.

 

If we're talking about one scene (and one scene only) that's essential to the plot, the answer is Walken in Pulp Fiction.

If we're talking about one scene that's a fun throwaway and otherwise unimportant, I'll go with Sean Connery showing up at the end of Robin Hood as King Richard. He's in the film for about 30 seconds.

 

Have not seen "Shooter" in a while, but IIRC Levon Helm's appearance was one scene and he was very good.

 

Richard Dreyfuss as a fisherman singing "Show me the way to go home" right before he's devoured at the beginning of "Piranha 3D" was pretty fun.

 

One scene that forwards the plot...

 

In the Dogtown skateboard movie (not the documentary), some of the guys were meeting astronauts. The astronauts were in their space suits. One of them tries a skateboard and almost falls on his ass. "This isn't as easy as it looks," he said. It was Tony Hawk.

 

Martin Sheen in the second Hot Shots! flick.

Bill Murray in Zombielamd.

 

Kurt Vonnegut in Back to School. All he does is say hello to Mellon’s kid at the door, but it turns into the fun scene when Mellon accused by the professor of not doing his own work and saying that whoever did write the paper knew nothing about Vonnegut.