First off, anyone putting Pettersson ahead of Gaudreau now because he's scored at a good (but significantly slower pace than Gaudreau has) this season over 30 games needs to check themselves. Gaudreau is already at Petterssons highest level. Top 5 in scoring and on pace for over 100 points, that's the best you can hope for any player and Gaudreau is there. You don't just put a rookie ahead of a player like that because he's played well for 30 games.
Second, Monahan is better than Horvat in every statistical measure.
Third, Tkachuk is a throw in here, we also have Lindholm who is as good as Tkachuk and Monahan and is the same age as Monahan and Horvat. Just a minor detail that is being ignored here.
Limdholm is not better than Horvat. He is benefiting from his linemates in Calgary. This year, Horvat has played almost exclusively on shutdown lines. He has played quite a bit with Roussel, Schaller, Eriksson, etc... He creates his own offence. He's on pace for 70 points and over 30 goals, without playing with Boeser, Pettersson or even Baertschi. Put him on a line with some better offensive players and he would get 70-80 points every year.
I know I might sound like an idiot because I'm a Canucks fan but Horvat is extremely underrated. I'm not saying he's a superstar like Gaudreau, but he just a tier below. His defensive game, leadership and work ethic are fantastic.
For me (right now) it's:
Gaudreau
Pettersson
Horvat
(Very small difference) Tkachuk
Monahan
Boeser
Btw I would take Calgarys young guys right now but it is close and I hope that changes in the near future