One definition of 'signature win': "a win that has people talking about it well beyond the normal football shelf life of seven days."
Another: "Beating a team ranked higher than us."
Yet another: "Shocking the nation with a win that is monumentally unexpected."
As you can see, this is another thread that really just boils down to semantics. How you answer the OP depends entirely on how you define the phrase.
I personally like the first definition best. It's fluid enough that it doesn't get bogged down in exceptions and one-offs, but still solid enough to provide a real measuring stick.
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Using that, here are what I would tag as the signature wins of each of Butch's first four years:
2013 - beating #11 South Carolina, 23-21.
2014 - beating Iowa in a bowl game, 45-28.
2015 - beating #19 Georgia, 38-31.
2015 (alternate) - beating #13 Northwestern in a bowl game, 45-6
2016 (so far) - beating #19 Florida, 38-28
2016 (alternate) - beating Va Tech 45-24 in front of largest crowd in history
As for the signature win of his career, I don't think Butch has had that yet. He hasn't yet met the third definition of "signature win," that monumentally unexpected win that shocks the nation.
It will probably come against Bama one day. Hopefully this December, but if not, more opportunities will come.
Go Vols!