I would start with Celebrini last. I think he just doesn't have the offense of the others. He'll have a good career, but I don't see him ever being a top 5 center in the NHL. I think the other three are true star game breakers that routinely are like 100 points a year at their best.
From there, you are dealing with different types of players. McKenna is the least athletically challenged. Yet, he's also the only one of the other three that isn't a center. Can he be taught to play center? Hagens I think is the best defensively of these three, but his offensive skills aren't as strong. Bedard is the shortest and worst skater of these three, but his shot might be the best attribute of these players and he's more proven. He's delivered in big moments for many years.
Potential:
McKenna (if he plays center successfully and improves a little defensively)
Hagens (if he grows another inch or two)
Bedard
Celebrini
Taking everything into account:
Bedard
Hagens
McKenna
Celebrini