Another thing to consider about BB's strength is also his body's physiology which WB stated as being abnormal. Is that the reason why he's able to duel wield devil fruits? Is that how he was able to survive Ace's onslaught of flame attacks sin their battle, even though he had the yami yami no mi fruit which doesn't make his body intangible like most other logia fruits and doubles his damage intake. BB's strength and power is still wrapped in a bit of mystery which makes it difficult for me to rank him with the others.
“Pirates are evil? The Marines are righteous? These terms have always changed throughout the course of history! Kids who have never seen peace and kids who have never seen war have different values! Those who stand at the top determine what's wrong and what's right! This very place is neutral ground! Justice will prevail, you say? But of course it will! Whoever wins this war becomes justice!”
― Donquixote Doflamingo