THIS article on Listverse. Whatever, my choice isn't even ON that list, which sidesteps the true answer by sticking to movies, tv and videogames only. But you see, the most badass, most epic sword hasn't successfully been translated into those media. You have to READ about it.
Obviously, that's Stormbringer, the soul-sucking blade wielded by Elric of Melniboné.
Now THAT'S a badass sword.