Okay, let's hear your ideas on who doesn't play their best CENTER on their first line. You could argue that Boston doesn't with Bergeron, but Kreijci and Bergeron are a 1a/1B combo. But ..... Where else in the league does this happen?
I'm not sure that them being a CENTER is a huge deal in this debate, on a lot of teams the first line center is the best forward on the team, therefore attracks the top defenders, thus making it the first line. However, in situations where the 1st line C doesn't define the team, there is a lot of question about who ices the best center with their best wings.
To save time since line combos tend to be a moving target on a game-to-game basis, I'm using this site:
http://www2.dailyfaceoff.com/teams/
MacKinnon v.s. Stastny? Debatable who's actually better with MacKinnon so inexperienced.
Johansen v.s. Anisimov? Debatable.
Barkov v.s. Bjugstad? Debatable.
I find this surprising, but is Plekanec playing on Montreal's third line? Though, looks like tons of weird movement in there to get balanced scoring.
Fisher over Legwand? Debatable.
With Stamkos down, Killorn with St. Louis over Filpulla? Not so sure they have the best Center at the top there.
Of note but not a center, looks like a Sedin got moved down to balance scoring on Canucks, pretty significant move to get a balanced attack?
The point being is that teams move forwards around to balance scoring, it's nothing new. It sometimes even gets hard to determine what is a "first", "second" line etc.. look at that montreal line-up for example.