Kate Winslet does the best Australian accent i've ever heard (or can recall) in Holy Smoke. I'd say you could count her mistakes in syllables on one hand. Very impressive, really made me take notice of her as an actor.
Anthony Hopkins does a good Kiwi accent in The World's Fastest Indian, though as i'm not a Kiwi myself, I might be misjudging.
To my Australian ears, Toni Collette does excellent American accents.
Hugo Weaving. Do I really need to explain? Even his Elfish Sindarin comes across as effortless.