Obviously take my opinion with a grain of salt (see avatar) but I think an argument could be made for anywhere from 3-6.
No question, McDavid and MacKinnon are 1 and 2. I think 3-6 mostly becomes a question of what you are looking for in a center. A pure offensive powerhouse? You take Draisatl. If you want unparalleled goal scoring ability, you take Matthews. If you're looking for an elite two-way shutdown defenseman, you take Barkov. If you're willing to sacrifice a bit of that defensive prowess for offensive skill, Pettersson is your man. It all depends on what your team needs.
Honestly, I think an argument could also be made for any of Hughes, Point, and/or Crosby in that 3-6 (now 9) range these past few seasons as well.
I'll make the argument that given the lack of "flaws" in Pettersson's game, he's closer to 3 than 9 but any of these centers could be (and in many cases previously were) the lynchpin to a Stanley Cup winning team.