At his old age whitebeard was still the strongest in the one piece vs him being that strong only shows how powerful him and Rogers was back in the days. He stopped a giant ship with one hand he soloed akainu when he was badly injured and even had his insides incinerated relatively quickly. It took akainu and aokiji 10 days to settle their fight whitebeard ended his in less then 5 attacks. After his fight with akainu he still almost split the island in half then fought teach. Whitebeard was unquestionably the strongest even while sick and in old age he can never be overrated he's a pure beast.