So according to the Ultra Analysis Book these are the rankings for the villains.
S-Rank Villains:
All for One
Gigantomachia
Tomura Shigaraki
Jin Bubaigawara aka Twice
Hood aka High-End
A-Rank Villains:
Rikiya Yotsubashi aka Re-Destro
Dabi
Himiko Toga
Chizome Akaguro aka Stain
Daruma Ujiko
Muscular
Nomu (U.S.J.)
Nomu (Hosu)
B-Rank Villains:
Kai Chisaki aka Overhaul
Hari Kurono aka Chronostasis
Joi Irinaka aka Mimic
Kendo Rappa
Kurogiri
Atsuhiro Sako aka Mr. Compress
Kenji Hikiishi aka Magne
Koku Hanabata aka Trumpet
Tomoyasu Chikazoku aka Skeptic
Geten
Moonfish
C-Rank Villains:
Shuichi Iguchi aka Spinner
Kagero Okuta aka Giran
Shin Nemoto
Hekiji Tengai
Rikiya Katsukame
Deidoro Sakaki
Toya Setsuno
Yu Hojo
Soramitsu Tabe
Chitose Kizuki aka Curious
Danjuro Tobita aka Gentle Criminal
Manami Aiba aka La Brava
Mustard
S-Rank Villains:
All for One
Gigantomachia
Tomura Shigaraki
Jin Bubaigawara aka Twice
Hood aka High-End
A-Rank Villains:
Rikiya Yotsubashi aka Re-Destro
Dabi
Himiko Toga
Chizome Akaguro aka Stain
Daruma Ujiko
Muscular
Nomu (U.S.J.)
Nomu (Hosu)
B-Rank Villains:
Kai Chisaki aka Overhaul
Hari Kurono aka Chronostasis
Joi Irinaka aka Mimic
Kendo Rappa
Kurogiri
Atsuhiro Sako aka Mr. Compress
Kenji Hikiishi aka Magne
Koku Hanabata aka Trumpet
Tomoyasu Chikazoku aka Skeptic
Geten
Moonfish
C-Rank Villains:
Shuichi Iguchi aka Spinner
Kagero Okuta aka Giran
Shin Nemoto
Hekiji Tengai
Rikiya Katsukame
Deidoro Sakaki
Toya Setsuno
Yu Hojo
Soramitsu Tabe
Chitose Kizuki aka Curious
Danjuro Tobita aka Gentle Criminal
Manami Aiba aka La Brava
Mustard