Mine is 4 which consisted of the following moves. e4-e5, Bc4-Nc6, Qf3-Nd4, Qxf7 mate.
Nifty little line and surprising how many people fall for it.
quickist checkmate
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It's called the 'fools mate' I once did it against my mum many years ago, on holiday. She wasn't very happy at all, I found it rather amusing though 
After all, she was the one who taught me how to play initially.
My quickist checkmate is 9 moves -