If we count God Valley annihilated by Imu to erase the existence of Xebec, Then there must be prohibited to say Xebec Existence... Even though He was Higher than Roger peak Bounty. Even though...
I pick 6B (Roku) for Rokkusu D Xebec. If we go nitpicking, 6.93B. He was the man whose 7 notable crews were troublesome like BM, Shiki, Ginfu, Ochoku, Kaido, WB, John.
Imagine: 6B Captain
# Buggy has 7 crews of 1B pirates like Beckman, King, Katakuri, Shiryu, Marco, Jozu, Lucky Roo
- Or -
# Buggy has 7 crews like 3b Mihawk, 1b Croc, 1b Hancock, 1b Jimbei, Doffy, Kuma, Moria
Well, 3B Luffy has 3 of 1B pirates
Yet, he was 6B but got into 0 B (the other name is 'xebec bounty wont be appear' because 'the great cleansing' agenda (an If)