Funny how people say in SS3 three hard slashes will kill your opponent. What they fail to mention is that it goes both ways, too. I lost to Zankuru after he hit me TWICE. I have SS3 on home cart. I love the game, but I've only beaten it once in roughly seven tries-on the lowest difficulty setting, using both sides of the controllers for extended credits.
Three takes this by far. I'd say SS1 is second hardest, then 4, then 2 (I don't find Mizuki as difficult as people say).