Roger & WB are Characters that Oda thought about from the Start
And it's expected that Luffy who represents Roger, should have his WB
Law & Kid were not part of Oda's Plan & were only Created after a Decade of One Piece
Teach is more like a Villain & not Friend Rival like Roger & WB were
So for me, it's pretty obvious that Luffy's WB is clearly Zoro, just because they aren't against each other doesn't mean they aren't.
For example, Sengoku is Garp's WB even though they work together
I know that Zoro have Parallel with Rayleigh but that's because both Luffy & Zoro can share Parallels with Multiple Characters, they aren't bond to just One. Luffy have Parallels with Joy Boy, Roger, Shanks, Oden, Rayleigh, Blackbeard ... etc while Zoro have Parallels with Ryuma, WB, Mihawk, Oden, Rayleigh, Kaido ... etc