MK9 is vastly different than the rest of the series, and a solid fighting game with great mechanics and a nice competitive community.
That said they have ALL been fun fighters more or less - stupid, but fun. It's the same reason that, say, Mario Kart has so much popularity; I don't think anyone is going to argue that the game is deep or anything, but it's a blast to play.
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Fresh from my first justified ban. Ever!