Miranda Lambert vs. Chris Brown: Nastiest celeb feud?

It was the tweet heard about the world yesterday -- Chris Brown wished Rihanna a "Happy Birthday," and she tweeted back, "Thanks!" Plus, the rumors were confirmed: Brown is featured on a remix of Rihanna's song, "Birthday Cake," and Rihanna is featured on a new version of Chris Brown's "Turn Up the Music."

SODAHEAD SLIDESHOW: See the nastiest celebrity feuds. 

Three years after Brown pleaded guilty to assaulting his then-girlfriend, it seems like the two superstars have officially patched things up -- but the one person who hasn't moved on? Country star Miranda Lambert.

"Chris Brown twice? I don't get it," Miranda tweeted the morning after the Grammys. "He beat on a girl ... Not cool that we act like that didn't happen." Brown subsequently tweeted in what was taken to be a response: "Hate all you want becuz I got a grammy now! That's the ultimate f--- off." The tweet was later deleted.

But Lambert didn't stop there. Before performing her domestic violence song "Gunpowder and Lead" last Thursday, she held up a neon green poster that said, "Take notes Chris Brown," E! Online reports. "I've been in a world of hurt with Chris Brown fans lately," she said. "But see, I just have to speak my mind because where I come from, beating up on a woman is never OK."

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Brown responded once again: "Using my name to get publicity? I love it! Perform your heart out!" he tweeted, adding, "Go buy @miranda_lambert album! So motivational and 'PERFECT'!" From Lambert and Brown to Charlie Sheen and Ashton Kutcher, let us know: Who had the nastiest celeb feud?