Q:

What is the most idiomatic translation of "no way!"

The phrase "no way" is similar to this question about the expression "you wish!" but is perhaps more of an expression of disbelief or rejection of what the other speaker says (short for There is no way that just happened!). It also can be friendly enough to be used jokingly or teasingly, like when another person says "Yes way" back and you say "no way!" again (like "nuh uh" / "uh huh").
I think I have heard just que no used - for example, perhaps something like this:

A: Cuando llegué a casa, el perro habia comido mi tarea.
  B: Que no!

This discussion on "a que sí" o "a que no", like some others I found while searching, makes it seem like "a que no" is the way this might be said, so maybe I haven't heard the "a":

If the conversation were such that the interlocutor disagreed with the assertion, the exchange would be as follows:
  A: He's really bright. -- Es muy inteligente.
  B: Yeah right! -- A que no!  

Is que no! a good translation of no way! or is there a better one?

A:

Actually there are a few possible translations given the context and country where you want to say "No way!".

¡De ninguna manera!
¡Ni pensarlo!
¡No hay el menor peligro!
¡Ni lo sueñes!
¡Ni hablar!

A:

Creo que la mejor traducción sería:

¡No jodas!

Pero depende del país y el contexto (no vale para todo y es una expresión muy coloquial)
En México:

¡No mames!
  ¡No me chingues!

A:

I've also heard ¡No me digas! used in this way.
This discussion, though not authoritative, somewhat supports this: http://www.quora.com/How-can-I-say-the-equivalent-of-the-Ironic-Spanish-Expression-ahh-si-no-me-digas-in-English

