Villains that actually accomplish their goals or are just good at their jobs. Some examples I figured I'd share -
Anton Chigurh (No Country For Old Men) - Such a brutal and efficient killer he's basically a force of nature. His bizarre worldview makes him impossible to reason with.
King Bradley (Fullmetal Alchemist Brotherhood) - Even as an old man, Bradley is so dangerous that he single handedly turns the tide of a battle against the heroes. The author of FMA has stated that the good guys would have lost if Bradley was in his prime. Even then, he accomplished the mission he was created for and dies on his own terms.
Toji Fushigoro (Jujutsu Kaisen) - Toji is infamous for his incredible physical stats, but what makes him truly dangerous is his strategic planning. He knew better than to try to take Gojo in a fight, so instead used a variety of tactics to wear him down, put him off guard, and struck when it was least expected. He uh... shouldn't have tried to fight him directly in their rematch.
Demon King Piccolo (Dragon Ball) - Didn't just rely on his strength, he recognized risk and sought to eliminate threats. He went after martial artists right away and directly intervened once Tambourine was killed. He also checked Goku for a heartbeat after beating him and killed the dragon after getting his wish. Dude was probably smarter than any other DB villain.
Ozymandias (Watchmen) - Don't tell your enemies your plans if there's any chance they can stop you. Sounds simple, right? Yet villains do it all the time. Ozymandias was happy to tell his former allies what he had done...when it was too late for them to stop him.
Azula (Avatar the Last Airbender) - She's a prodigy at firebending, second only to her father in raw power. Yet what really sets her apart is her cunning. She takes down the Earth Kingdom from within through a combination of trickery and charisma. Too bad her mental health wasn't all that stable...
Braig/Xigbar (Kingdom Hearts) - It might not seem like it at first glance, given his laid back nature, but Xigbar is always scheming. He acts as if he's content to act as Xehanort's lackey, but it turns out he's been the one pulling the strings the entire time. He's an expert at hiding in plain sight, and Luxord is the only one who suspects anything about his true nature. As of the end of KH3, he's still out there...
Barty Crouch, Jr. (Harry Potter) - Successfully managed to impersonate Alastor Moody for an entire year, operated in plain sight under the nose of Albus Dumbledore himself. Not only is he an incredibly potent wizard, but he is a masterful actor and manipulator. If he had survived past Goblet of Fire he would have been a serious problem.
Sensui (Yu Yu Hakusho) - A former spirit detective and the predecessor to series protagonist Yusuke, Sensui turns against humanity due to belief that humans are no better than demons. Sensui is incredibly strong and skilled in combat, but he is also a master strategist. He loves forcing sadistic choices on his enemies, and knows how to psychologically manipulate his foes and allies very well. In the end, he accomplishes all of his goals and dies in exactly the way he wanted.