Pharah doesn't need a nerf though, she's perfectly fine. The only reason why she was so heavily used beforehand is that her counters sans 76 weren't all that good in general, but now there's the buffed McCree and Ana as well to counter her.
How do you determine who has "better stats" then? Sure, for an offense character that's pretty simple, damage done/kills/final blows are easy to track, but what do you do for supports or tanks?
A Lucio who has better healing stats is not necessarily a better Lucio than one with lower ones - perhaps the one with the lower healing stats knows when to switch to speed boost better, for instance. Same goes for instance for a Reinhardt - more damage blocked doesn't necessarily mean he is better at tanking, perhaps he's just one of those Reinhardts who have their shield up all the time and aren't strategically using it, or do nothing but shield.
Do you take average win quote? Would that really be fair, though? Someone preventing you to play a character just because they won more often with that character? Note the fact that Overwatch is not a single player game, but a team game - someone winning more often playing a hero does not necessarily mean they are better at playing that hero than someone else.
I don't think it's possible to determine who is better at playing a specific character through a numerical way, unless it's a massive difference in metrics.