Tbf it's a very debatable thing and no one option has been set in stone like say Zoro's, Chopper's or Ussop's case. Personally, I still think it's Senor La Feet though there are pretty convincing arguments on other matchups.
Yeah, but it's fairly easy to eliminate certain members. Oda has a habit of matching up Zoro and Sanji with characters who share a common theme with them. On a surface level, Pizzaro seems the most likely candidate.
I'm of the mindset however, that Aokiji is not actually the Tenth Titan commander referenced in Dressrosa, and Oda is playing off of Sanji seemingly going unnoticed by the WG all of preskip, and parallel this mysteriously unknown Blackbeard Pirate member, to be his opponent.