This isn't based on "strength" but character.
Upper Moons-
Akaza- Amazing character
Kokushibou
Gyutarou & Daki (heart wrenching backstory)
and then way off in the distance-
Douma- second only to Muzan in "evilness", so can't say I "like" him.
Gyokko
Hantengo
Hashira- I love them all, but
Giyu
Gyomei
Tengen
Rengoku
Tokito
Mitsuri
Shinobu
Sanemi
Obanai