As a long-time One Piece fan, it's hard not to automatically lean towards 'Monkey D. Luffy' as the strongest character. I mean, you can't forget his Gear Fourth Form and those King Kong punch moves, it's next level awesome! He's continually pushing his limits and overcoming obstacles that seemed insurmountable.
But at the same time, we cannot ignore the raw power of characters like 'Whitebeard' or the Yonko 'Kaido', who are both literal powerhouses. It's quite a debatable topic.