Stat wise: between rubedo and gargantua. Skill and usefulness wise: Ainz. Pure dps probably between Ulbert, touch me, nishikeinrai, and warrior takemikazuchi.
rubedo
touch me
ulbert
ainz with full equipment
there are some unknowns like the 8 hierarchy, gargantua ect
G
never said that m8
“Rejected, Ainz answered immediately, Nazarick’s strongest individual, Rubedo. Her battle potential was well above that of Sebas, Cocytus and Albedo in terms of pure melee combat, Shalltear would be a weakling in comparison, given the fact that even Ainz in his full equipment would most likely loose against Rubedo, the only few that could possible win against that one would be those stationed on the 8th floor and only if they were to use world class items, as expected I don’t think they could put up a fight against that individual otherwise,”
Rubedo
Don't forget this one important fact about Ainz: he always underestimates himself, to the point of self depreciation. Until it actually happens, I'll never take anything he says at face value.