I'm not so sure that Marner gets the edge in pure, hypothetical upside. MacK at his best though is an unstoppable freight train out there and Ekblad at his best is a dominant defender. None of them has come close to topping out, it's just a matter of who has the right situation, health and luck to maximize their talents.
Now in terms of intelligence and vision, Marner's the pick of the three and I can see why that would lead to some confidence in his future performance returns. He's just a smarter player than the other two - who are more physical specimens.
I definitely homer voted but you can make a case for all of them.