Black Shadow, easily. He's the most notable villain F-Zero has ever had, plus the scariest racer: in GX, Black Bull is one of the best machines in the game, his AI is stronger than any other, and he's the boss of the most difficult race in the story mode. He's also the main villain of the anime, and enjoys a lot of popularity in Japan thanks to his voice actor. Sure, his character design is pretty ridiculous, but that's F-Zero in a nutshell, plus I think that would just make him all the more iconic to Smash fans being introduced to the character (much like Falcon in 64 with his ridiculously loud/flashy style). That he is a perfect home for Ganondorf's old moveset (which I personally like and would hope stays around) is just icing on the cake.
On the flip side, there's Goroh. What's notable about him? Well, he was in the original four, plus he was an assist trophy in Brawl, so that helps for sure. On the other hand, from my biased GX point of view, his machine is one of the worst in the game, his AI is also among the worst, he's the boss of one of the easiest story chapters, and he generally comes off as a joke of a character who isn't very competent. As for using a katana, isn't that why everyone wants Takamaru? If they were to add fire just because he pilots Fire Stingray, why not Roy? I don't see what makes him so special moveset-wise, at least compared to other popular picks.
To be honest, I kind of don't want to see a second F-Zero character at all for SSB4. The franchise has also been dead outside of Japan since GX nearly a decade ago, and dead there since Climax almost nine years ago. The time just isn't right for more F-Zero representation. Still, should we get so lucky, make it Black Shadow. An easy late inclusion with a built-in moveset and plenty of notoriety within the franchise to deserve the spot. Goroh, by comparison, is too much work for a character that just isn't that interesting or even that important to the series.