Soul Calibur X Square Enix
I can't really think of anyone who bought SC4 cause Yoda or Vader were in them(actually alot of my friends who casually dabble in the series, laughed it off as a joke). Nor Spawn or Heihachi. The most successful guest character was Link. I remember people picking up SC2 just because "Link is in a REAL Fighting game!"(hilariously enough, thats how I got into the series. Link's announcement hyped me pre-console release, which spark my interest to buy a Dreamcast to play and learn SC1 to prep.) of course out of those three, Link was the worst. But it didn't matter. Link = sales. And the GCN version sold damn well.
Which looking back actually ASTOUNDS me that they made SC3 PS2 only(and console release first, but thats a different subject entirely). I know FGs on Xbox Live was just starting to pick up then(they weren't ready to attempt to take the game online, if I remember) and the GCN version, with Link's inclusion sold hella more than the other 2.