When it comes to determining how strong a champion is which matters more? Take Leona who has a 53% win rate but only a 0.8% ban rate or Malzahar who has a 52.5% win rate but just a 0.4% On the other hand look at Leblanc who only has a 46% win rate but a 11% ban rate or Jayce with a 47% win rate but a 7% ban rate.
ban rate = not played win rate x (1 - ban rate) + theoretical win rate x ban rate = true win rate.
Assuming theoretical win rate = 1 (whoever picks this champ will 
tee a win!)
Result:
Leblanc: 0.46 x 0.89 + 0.11 = 51.94%
Malz: 0.525x0.996 + 0.004 ~= 52%
Jayce: 0.47x0.93 + 0.07 = 48.83%
Fault of this approach: Skews data to favor whoever has higher ban rate
Assuming theoretical win rate = 0 (whoever picks this champ will tee a loss!)
Result:
Leblanc: 0.46x0.89 = 40.94%
Malz: 0.525x0.996 ~= 52%
Jayce: 0.47x0.93 = 41.83%
Fault of this approach: Skews data to punish whoever has higher ban rate
Assuming theoretic win rate = 0.5 (average players will always pick this champ) Result: Leblanc: 0.46x0.89 + 0.11x0.5 = 46.44% Malz: 0.525x0.996 + 0.004x0.5 ~= 52% Jayce: 0.47x0.93 + 0.07x0.5 = 45.33% Fault of this approach: Assumes average win rate for that particular champion (does not account for meta, steeple, pro-player streams, etc.)
Overall ban rate doesn't affect the true win rate comparison much... I suggest looking at their play rate! (how often that particular champion is actually played will obviously factor into that champion's strength)
imo, win rate is what matters. People tend to not recognize "invisible power" For example, people who provide auras tend to be unappreciated, and assassins tend to be loathed. As such, people (ban rate) aren't indicative of power.
