The six Rokuō are not ranked in any particular order, as each member of the six-way balance holds his position by meeting certain criteria which are not necessarily fixed. Nixa and the Maju Army, for example, are considered to be part of the six mainly through the strength of Nixa and the military and arcane prowess of his (extremely large) army. On the other hand, the Ten Grandmasters are less powerful in terms of raw strength, but are able to maintain their status as Rokuō through their highly respected reputation as peacekeepers and men of great skill, as well as through their connections with the Jiang Dynasty (the fall of which now threatens their position as Rokuō). Ludwig, though already a powerful entity in his own right, maintains his Rokuō status through the Multiverse's heavy reliance on the Hotel which he created, as well as through the influence each of his seven Tenshi hold.
The Black List, on the other hand, is more straightforward in its criteria; any individual who is infamous and holds a track record of military success can be considered for a position on the Black List, provided that their K.O. count is surpassed by no more than 19 other individuals. Unlike the Rokuō, however, the members of the Black List generally do not wield massive influence or political clout per se, as they prefer to work either independently (as is the case of Adam Azorug AKA Mu Balakas, who seeks to depose Ludwig to ultimately reestablish the House of Balakas) or as mercenaries (like Anneliese E. Jones and Snakyashi). The exception to this is Iuzun Nebra, who not only holds the #1 spot on the List (for 17 years and counting), but also rules as a Rokuō through the IGSE.
As such, there are warriors who wield more power than Black List bounty hunters, but are not considered to be among the twenty; an example is Danashili the Medusa, whose K.O. count exceeds a good number of Black List bounty hunters, but is not among the list due to her status as a direct subordinate of Nixa (and not a mercenary). Likewise, some of the Black List bounty hunters are more powerful than individual Rokuō, but are not deemed as such because they lack influence, political power, or reputation outside of the underworld. Omo Majar Senga Duma may command the evil Toto Army, for example, but said organisation is, in terms of influence and power, nowhere near any of the Rokuō.
In all truth its also advancing some story plots as well.....