I think for a lot of Nashville fans, the luxury of having these four defensemen, especially seeing the group take the team to the SCF in its first year together, causes this weird problem of rotating out who is the best defensemen on the team at a given time. Normally you have your top guy, who may end up in competition for "the guy" with a clear 2nd guy. If you're lucky, the 3 and 4 positions are either ambiguous in a similar way to the top pair or at least filled by clear 3 and 4 spot guys, but there is almost always a noticeable gap between the top and second pair. Then, your third pairing guys are either kids who aren't ready for heavy minutes but need to continue their development in the NHL, veteran guys who are there to steady the ship but generally aren't going to play many dangerous minutes on purpose, or a combination of both.
So in Nashville there is this rare, arguably unique, situation in which the differences between the top two pairs have more to do with style than talent and skill. You have two guys who are legitimately #1 defensemen, Josi and Subban, playing on two different pairings with defensemen who are legitimately 2nd pairing defensemen who are bordering on being top pair guys. Great stretches make different guys become "the guy," and poor stretches make them fall off in the minds of the fans.
With Subban I think it takes him longer to recover the trust of the fans after a bad game or a bad stretch. There are many reasons that could be true, not the least of which is leftover feelings about the Weber trade. His animated personality is something people can easily point to when he makes a mistake, claiming it as proof that he isn't focused as much as he should be (during the playoffs it was either JR or Milbury who claimed Subban wasn't focused because he was dancing to a song during warm-ups). He is polarizing because he doesn't act like the stoic NHLer people have been used to, and, let's be honest, he does do some stupid, pest-like things during games that would piss me off if he weren't on my team. Those things will lead to penalties, often at very bad times, and he catches hell for that (deservedly so).
There are, objectively, two guys one of whom is the best defenseman on Nashville, just in terms of player ability. Those guys are Josi and Subban. It becomes subjective at that point and, despite what some might try to pass off stats-driven, objective fact, a lot of the decision then is a matter of taste or a matter of situation.