The heroes in Titans never have a slow day. There's always some new bad guy scheming their way to glory (or following orders from a more influential villain). Most of the villains are plucked directly from the pages of DC comic books, with the majority having made their name through interactions with Golden Age heroes such as Batman or Superman.
While all the Titans baddies so far have proven to be very good at inflicting pain and damage, a few have stood out for the better use of their brains. These characters have either come up with superior schemes or were shown to have a naturally high IQ. Even among the smartest of them, there's still a hierarchy of brainpower.
Nick Zucco (The Melting Man)
Nick grows up in the mob as the son of Maroni crime family Tony Zucco but opts to be a serial killer rather than an organized crime member. After his father's murder, he goes on a mission to wipe out members of Haly's Circus.
Nick's frenzy makes him an entertaining character, but he operates on nothing but rage. His choice to go after the entire circus is a misguided one since it's Dick Grayson he has a problem with. He has no concrete plan for his revenge either, and that's why he is easily stopped by Jason Todd and placed under custody.
Dr. Arthur Light
Light gains superhuman powers after an experiment meant to manipulate light goes wrong. The Titans arrest him and place him in the San Quentin State Prison, but he escapes. Eventually, he is killed by Deathstroke.
The physicist has a doctorate from the California Institute of Technology. This, combined with his dangerous experiment, confirms that he is a genius. Unfortunately, he isn't as sagacious away from the lab. His achievements in science have made him arrogant hence he never takes time to anticipate enemy moves. When he gets arrested and late killed, it doesn't come as a shocker.
Lady Vic
The assassin Lady Vic has a grudge against Barbara Gordon for the death of her partner. As she seeks vengeance, she agrees to work with Jonathan Crane to help him make his plans a reality.
In most of her scenes, Lady Vic is portrayed as a skilled fighter more than a thinker but she has her brainy moments too. One of those is when she uses a recording of Bruce Wayne to lure Barbara Gordon into a trap. Additionally, she is great at hiding from the authorities and for this reason, she has never been caught, despite committing hundreds of murders.
Komand'r (Black Fire)
Kory's younger sister has a tough childhood where she is constantly ridiculed. As an adult, her fortunes change significantly. She becomes the Queen of Tamaran after murdering her own parents.
Komand'r's intellect is best demonstrated by the way in which she manipulates her way into power. She mind-controls Kory and Faddei, making them oblivious to her plans. And when she lands on Earth, she proves to be the master of disguise by using tamaranean-tech to possess a woman's body, thus making it easier for her to bring mayhem without being noticed.
Jason Todd (Red Hood)
There are many similarities between Jason Todd in Titans and in the comics. He succeeds Dick Grayson as Robin but grows disgruntled while working with Batman and the Titans. After the Joker murders him, he is resurrected by Jonathan Crane at the Lazarus Pit, becoming the Red Hood.
As a villain, Todd finds clever ways to earn instead of simply stealing, He manipulates mobsters into giving him 40% of their earnings by murdering a few lieutenants and threatening to take out even more. He is also quick to realize that the anti-fear gas is what's corrupting his mind so he stops taking it. He then does the right thing by helping defeat Crane and ultimately retiring as the Red Hood.
Mercy Graves
Mercy becomes Lex Luthor's Personal Security Specialist after joining the Cadmus Laboratories. She is then put in charge of a manhunt to find Krypto and Conner after their escape from Cadmus.
Mercy has great attention to detail, a trait that stems from a high IQ. It's through observing little patterns that she is able to track down Krypto and Conner. Her stealth proficiency is remarkable too and that's why she sneaks about on Conner easily, despite him having superhuman senses. So far, she's the only person who has caught him off guard.
Slade Wilson (Deathstroke)
The Titans iteration of Slade Wilson, one of many on-screen versions of Deathstroke, served as a commando in the U.S. Army's Delta Force Unit where he underwent bio-enhancements. He then encountered the Titans and killed Garth, an act that created enmity.
Thanks to him undergoing enhancements as part of the Super Soldier program during his time in the military, Slade's brain can process information much faster. This is best seen during the fight with Dick and Cory when he tailors his tactics to suit each hero. He is quick to figure things out and is able to discern that his son is a meta.
Trigon
The demonic being Trigon is one of the most powerful characters in Titans, and he's keen on becoming the ultimate ruler. He is worshiped by a cult of acolytes and sires Rachel through one of them.
Thanks to his ability to cast illusions and gaze into the minds of humans, Trigon has grown to be a master manipulator. He convinces numerous acolytes that he is a decent being worth worshiping, and they agree. By taking on a human form, he constantly fools people that he is harmless, yet he is highly treacherous.
The Joker
The Joker first gets arrested and taken into the Arkham Asylum after shooting Barbara Gordon. He orchestrates a prison break and beats Robin to death with a crowbar before an angry Batman shows up at his cell and bludgeons him to death.
The supervillain doesn't get to do much on the show, but he is described as the most successful criminal in the history of Gotham. It's a huge achievement since the city is populated by all kinds of criminal masterminds. The manner in which the Joker taunts both the police and Batman after killing some of the city's most influential people shows he knew what he's doing. And it's only him that drives Batman so much to the edge that he breaks his "no-kill" rule.
Jonathan Crane (Scarecrow)
Dr. Jonathan Crane spends many months as Inmate #36215 at the Arkham Asylum as well as a consultant for the Gotham Police Department before escaping. He then begins working with Red Hood to produce the anti-fear gas.
Jonathan's status as an inventor and pioneer puts him high up the intelligence list. It's through vigorous research that he is able to create the fear gas. He is great at recruiting allies too. During his few interactions with Jason, he realizes that they think alike hence when Jason dies, Crane resurrects him and makes him his sidekick. One of the reasons Scarecrow is arguably Batman's greatest foe is shown in the series too. He successfully takes over Wayne Manor, turning it into the Crane Manor, something no one else had managed before