I think the most honest way to determine the best athlete on the field is to introduce race as determining factor. Yes, race. Not racism, people. The position that is strictly dominated by one race because of it's strict physical demand will tell you where the best athletes are.
The positions that are physically demanding and strictly dominated for the most part by one ethnic group are(in order):
CB -> RB -> WR.
WRs. lately we've seen some good #2 WRs with some multi-ethnic representations. But, unless by default, all the #1s have been one ethnic group dominated.
RBs. We've seen some rare sightings of multi-ethnic representation coming of the bench. But, unless default, all the starting RBs have been of one ethnic group.
CB. I would say, that's the only position that is strictly dominated by one racial group because of its strict demand of speed and the utilization of that speed.
So, CBs are the best athletes on the field.