There's no doubt Cowher was a fine coach, during his tenure with the Steelers. However, do you feel he was the best ever? Personally, I do. Think of how many years the Steelers made the playoffs under his watch. Even with only 1 Superbowl win, he kept his team as top contenders year after year. That has to mean something!
I agree, I don't think either is "the best ever" as each had his own strengths and weaknesses. I think they were both fine coaches and did alot for the team. They left quite a legacy for future coaches to follow.