Mockingbird.
I have seen them chase birds of prey, which were many times larger, away from their territory. I've seen them attack cats, too. Hell, I've been attacked, harassed and told off by mockingbirds, before.
Everyone hears them "singing sweetly," imitating other birds calls, but interestingly, the mockingbird's "own" sound is a hissing growl that sounds more like a reptile than a songbird.