I'll keep Mitch given our build and his current fit. Although, I'd expect Barzal to currently be the consensus winner. But, if I'm losing Marner I seriously look at Provorov and Werenski close to Barzal.
The Marner in the back half of this season was truly elite and looked comparable to elite players like Giroux, Kane and Gaudreau. His percentages ran a bit high post-All-star game, especially on the PP (19% oiSH), but he also gets below average PP time for a guy who is as good as he is at it. I'm very curious to see what he can do over a full season, playing with one of Matthews or Tavares. He exploded last season once he got put with Kadri instead of Bozak.
Now, if Arizona could redo the pick, I'd expect Barzal to be the answer.
I'd add, the list should be at least 6 deep with an argument for 8 deep. Rantanen absolutely deserves to be in this discussion, Aho is more debatable but probably also belongs (although, putting Aho in, probably brings up an argument for Kyle Connor). Either way, the 2015 draft looks to be the best draft since 2003. Most people should be fairly happy with their picks. That's before even getting to other NHL contributors who don't belong in the discussion, but their teams are happy with like Debrusk, Carlo, Beauvillier, and Konecny. And then there's guys who looked good but were either sheltered or didn't play the whole season who teams are optimistic about like Dermott, Dunn, Malgin, Roslovic, Sprong, etc. There are also guys who are productive but not really living up to their picks like Zacha and Meier. Then there's a kid like Kaprizov who has the makings of an impact NHLer but is over in the KHL.