Not a movie but Agatha Harkness from WandaVision
HIVE could qualify as the big bad of AOS due to being responsible for Hydra. Can't really think of anyone else.
Manny would qualify though IMO
Grand Moff Tarkin. He ordered the deaths over 10 billion people (2 billions on Alderaan, 8 billions of Geonosians, thousands of imperials and rebels on Scarif, Aresko and Grint etc.)
The Flash definitely couldn't happen without Thawne as he killed Barry's mother and created Flash as well as many of his villains.
Who would be stupid enough to work for Joker? He regularly kills his own henchmen for no reason.
My favorite villain redemption is probably Agent Kallus from Star Wars Rebels mostly because of how much time they spent with it and how they turned a sadist one dimensional villain into a symphatetic character.
My least favorite is Bo-Katan Kryze because she never even acknowledges that all the awful stuff she did with the Death Watch and because she only really turned against Maul because of racism which isn't a super relatable thing.
Theo Galavan (Gotham)
Cesare Borgia (Assassins creed)
Barry Allen from the Arrowvers indirectly killed Zoom
Theo Galavan from Gotham
Well first of all all of the other PE ones like Phasma Vitiate Pong Krell Tarkin Krennic Nuvo Vindi Keeper Argyuss etc.
To be honest i think Sauron is pretty boring he has barely any personality
Eli Mills while he is not as heinous as Dogdson the movies are a separate canon and he is by far the most heinous villain in the movies.
Despite him not being the "Real" Joker he still was very obviously a version of him
They were both great but i prefer Jerome, he was a total monster but you couldn't really hate him because of how energetic and fun he was
Penguin from Gotham
Happy Birthday
They are very interesting
Mr Burns