I think Couturier has the best chance of being the best player, and is the safest bet to be a top line center. Not saying someone else might not end up better (same would be said for all prospects), but that's who I'd take.
He's been the top point producer and best offensive player in this draft class for two straight years. Think about that in the context of draft history.
Look at other guys who were dominant a year before their draft year, and most of their production only went up a bit in their draft year. Specifically, guys like Crosby (17%), Stamkos (15%) and Hall (8%) all saw modest production gains versus their draft year peers, many of whom more than doubled their production. But it didn't mean they weren't the right "first pick". A guy like Tavares' production actually dropped in his draft year, and he likely only saved his 1st overall status with a stellar WJC (but not without argument).
For Couturier, his production increased by 17% this year on a ppg basis, despite having a worse team and mono to start the year. All this, and he was still the top ppg player in the enture CHL (let alone his draft class). Everyone says their are concerns about upside, and having watched him, I don't get it. He is a dominant player.
I know people point at numbers, but offensive players are defined by production, and there have been none better than Couturier. As such, and based on watching him play, I think he's the safest bet to be a premiere offensive centerman. There is no way this guys development has plateaud, anymore than it did for Crosby, Stamkos, Hall or Tavares in their draft years.