Without going back to the history books I think they won 4 AAFC titles in a row 1946-49, twice going undefeated!
When the AAFC was gobbled up by the NFC I think Cleveland, San Fran LA and one other team came into the NFL. No one gave them a chance but they won the NFC championship the very first year in the league & did win 3 more, incredible.
Otto Graham retired for a year & they lost that year, but Paul Brown persuaded him to come out of retirement for just one more year and he led them to another championship.
Otto Graham & Paul Brown stand alone, no question about it