As far as I understand rubedo is strongest npc while shall has highest stats and is the strongest all round character in the floor guardians. Mare has the second highest stats and is the strongest pure magic caster. Cocytus, Albedo and Sebas have a Rock Paper Scissors with sebas having the best pure fighting technique, Albedo having the best defense in nazarick, and cocytus being the best fighter with weapons. And aura is the best group fighter with all her pets. Pandora’s actor is somewhere in there based on who’s form he is taking. Not much is known about the floor 8 creatures so we can’t really compare them.

shalltear is the best in 1v1s (not counting rubedo) she can 1v1 any floor guardian except demiurge and she's countered by aura

sebas is the best in terms of unarmed combat (not counting rubedo), he counters he cocytus but is countered by albedo

albedo is the best with in terms of defense she is countered by cocytus and counters sebas, ainz called her the strongest of the 3 because between the three she has the highest stats at 670 so that acts as a tie breaker

cocytus is the most skilled with armed combat being able to use 4 weapons at once

and rubedo cucks them all. she outclasses them in terms of warrior capabilities period and in terms of stats. shes a monk like yuri so she is actually the best with unarmed combat not sebas and she has been stated to be able to beat shalltear so shes the best in 1v1s for sure

How can she not 1v1 Demi? Is he too smart?

He specialist in commanding strategy