He's not even the best now. Baker Mayfield had better numbers this year.
Looking at another Browns QB, what could Otto Graham have accomplished if he had today's training and nutrition, could play 16 regular season games a year, and made enough money to stay in the league more than 10 years? Championship games played in prestine conditions? Would he be better suited to today's game than the 40's and 50's? What about Johnny Unitas had those conditions?
So many "what ifs", so many changes in the game, it all comes down to opinion. And opinion is often formed by what was seen most recently (or at all).