In the Tower there are three different kinds of Ranking. There is Ranking and Grading with which Regulars and Rankers are ranked separately, where one's overall rank is dependent on their Shinsoo Resistance, Shinsoo Control, Intelligence, Weapon Mastery, Weapons Level, Willpower, Potential and their weapons and Pockets. (Note: Stat Card rankings were dropped by SIU and are therefore incorrect.) Then there's the ranking of the Rankers which is decided by the Ranking Administration Office where one is ranked according to one's influence and power within the Tower; the top 1% of Rankers are High Rankers and are given a sobriquet.
Finally, there's the Regulars Ranking which is dependent on what Floor one is on. We know E-ranked Regulars become so after passing the 20th Floor and D-rank Regulars become so after the 30th Floor and are free to participate in the Hell Express ride. SIU confirmed the existence of D, C, and B-ranked Regulars and recently A-ranked regulars, though it is currently unknown what Floor you have to be on to achieve the latter three ranks. It is unknown if there can be S-rank Regulars.
Possibly the highest rank a regular can obtain, above B-rank regulars.
Are Regulars that have transcended the strongest of the strong individuals of D and C-rank Regulars.
Are all Regulars who passed the 49th Floor.
Are all Regulars who passed the 30th Floor.
Most Regulars that come into the Tower on the Second Floor.
Regulars with Known Ranks