Joyboy > Roger >= WB >= Xebec
But Xebec was indeed a monster
Oda simply introduced this character saying it was needed Roger and Garp team up to defeat him
Then he show us that all that monsters like WB, BM, Shiki and Kaido were his crewmates
Then he said that all of these toptiers hated his captain, but still stayed in the crew. Why? Because they knew that Xebec was freaking monster, a guy in another level.
Oda simply showed the feats of all these top tiers along the series and said "hey, u know all these monsters like WB, Kaido, Shiki and BM, well they all were below a certain mysterious guy"
Xebec simply has the aura to be above Roger and Whitebeard