Superb Lyrebird (Menura novaehollandiae).
Here in the north eastern United States, our best mimic is probably the Northern Mockingbird. Don't get me wrong, these birds can belt out an impressive array of vocalizations from other species, but neither these, nor the Brown Thrashers, nor the introduced European Starlings quite manage to pull off some of the fancy vocals uttered by the Lyrebirds.
Here's a video I recently came across (by way of Gunnar Engblom of Kolibri Expeditions) showing just one example of Lyrebird mimicry.
A member of that overly large order Passeriformes, the Superb Lyrebird is one of only two species in the family Menuridae. Found in the coastal forests of eastern Australia, they're not only awesome mimics but they're also known for having one of the loudest vocalizations of all birds, and for the extravagant courtship display of the males. In addition to vocal flourishes that make even Whitney Houston sound monotone, the male raises his long, ornate tail feathers up over his body into an arrangement of feathers resembling the bird's namesake, the lyre. Pretty hot stuff if you're a female Lyrebird, but for the rest of us the vocals are the real showstopper!
If any of this is sounding familiar, you've probably already seen this specie's appearance in Sir David Attenborough's BBC series, The Life of Birds:
Perhaps you haven't seen the rest of the BBC footage... the really good stuff? I mean, I'm sure nobody would ever be tempted to exaggerate this birds already amazing talents. Would they?