Who does a better American Accent, she or Toni Collette?

As an American, I think they both do great accents, but they have very different voices. Collette has a higher more nasaly voice, while Griffith's is a bit lower and more mellow. They both sound authentic though in everything I've seen them in. Griffiths' accent on "Six Feet Under" was consistently impressive, albeit a slip-up or two, but she sounds genuine. It's been awhile since I've watched Collette in anything so she's not as fresh in my mind. The thing about Australian accents, I think, is that they are very strong, so when an Australian actor does slip up, you really hear it or it's usually easier to tune into. There was only one or two occasions in "Six Feet Under" that I recall Griffiths reverting back to her native tongue and she snapped out of it fast, but it was very distinct and noticeable when it happened. I'm sort of surprised the editors didn't do something about it, but honestly I think that a lot of people wouldn't hear it unless they were paying close attention. Some people watch things with one eye and one ear, you know?

But anyway, my two cents— I think that if I didn't know that she and Collette were Australian, I'd probably never guessed from having watched any of their performances as American characters.

Definitely Toni Collette.