Personally i think the list would go:

20) rigrit bers caurau, confirmed to be stronger than PDL

19) Deep Darkness Dragon Lord, Beat a player 1v1

18) Zy'tl Q'ae, required all 13 heroes to beat

17) Aura, stated to be the second weakest floor guardian

16) 7th Floor Area Guardian

15) Demiurge, stated to be one of the weaker guardians

14) Albedo,ainz said she might be able to beat sebas

13) Cocytus, beat albedo

12) Sebas Tian,beat cocytus

10) Warrior Takemecazuchi

11) Mare, third most powerful floor guardian

10) Momonga

9) Shaltear Bloodfallen, second strongest floor guardian

8) Tabula Smaragdina, stated to have more power than ainz

7) Pandoras Actor, (WHEN IN TOUCH_ME's FORM)

6) Gargantua, strongest floor guardian

5) Ulbert, as strong as touch me

4) Touch_Me, third strongest warrior in yggdrasil

3) Rubedo, stated to be the strongest in nazarik (by far)

2) the corrupted world champion, became a world boss

1) Penguin Butler, The One True Overlord , nuff said