Back Dimensional Count Radiguet is often regarded to be the worst Complete Monster in Super Sentai history (his only competition is this area are Long and Basco, see them below), Radiguet is a sadistic Ax-Crazy genocidal maniac with megalomaniacal fantasies of ruling the Vyram under an iron fisted dictatorship. Even when Juza, his boss and the true Big Bad, arrived he planned to overthrow her but that failed. Afterwards when it looked like he would become good because of a sick girl (who became his Morality Pet while he had amnesia) whom he cured unknowingly he spurned her offer to turn good by killing her. All because she believed he could feel love. He's also the one who brainwashed Rie into being Maria for no other reason For the Evulz. His most repulsive Kick the Dog moment was when he killed her after she snapped back to being Rie. There's also the part where he tortured ]Tranza till he became insane and was left at the door of an asylum.
Colonel Shadam is a nasty version. Responsible for many bad things in the series on top of the fact that he cares nothing for his two sons and wife, his amazingly abusive treatment to Akomaru is the ultimate proof of this. He shows zero honor at that: when Doushi Kaku showed mercy for him and the tables were turned for his side, Shadam instead uses the chance to mercilessly and brutally murder Kaku through an utter Curb-Stomp Battle. He also doesn't give one freaking shit about any of his subordinates, and the fact that \they are clay dolls created by him and he plans to discard them once they are useless to him make this even worse. Even worse, that includes his apparent master. Yes. This guy gives no shit about his friends, his child, his master, or his whole race. It's all about him in his head. It's particularly cathartic when he met his demise...and gets revealed that he toois a clay doll created by the original Shadam thousands of years ago and crumbles while begging Ryou for help...
Commander Guirail is only a newcomer, but he's a seriously depraved creature with love of underhanded tactics, whatever it takes to win. Even children wouldn't be spared. His favorite tactic seems to be 'Human Shields', and anyone's fair game. Even his ally Yugande, and after 'tricking' him to work together for the common cause, as well as making him take full brunt of damage when the chips are down against him. He's the only one amongst Nejirejia Kingdom who knows nothing of 'family/companionship' value, the moment Yugande was subjected of those things, both Hinelar and Shibolena were thoroughly disgusted and socked him with a Psycho Serum under the guise of power-up in revenge of what he did to Yugande. Many would agree that would've counted as Kick the Son of a Bitch rather than how depraved Hinelar was.
Grand Witch Grandienne, who deliberately sent her children out to die, brainwashed one to kill another on several occasions, banished one to Hell, and resurrected two of them as brainwashed zombies. As fitting of the family theme in GoGoFive, Grandienne stands for everything the Tatsumi family bond isn't. She has to be killed multiple times; apparently, just one Karmic Death was too kind for this monster.
Captain Ryuya, Tatsuya Asami's Identical Grandson (give or take a few dozen 'greats'), was the ultimate mastermind behind everything that occurred in the series — from the initial escape of Don Dolneiro to Gien's insanity. In other words, he was responsible for the crisis that took place in the finale (he had seen two futures — one in which 1/3 of the 21st century is destroyed and one in which the 31st century ceases to exist). To make matters worse, the only reason he was trying to manipulate everything was that he had received a vision that he would die in both futures.
Dezumozorlya is this, an Eldritch Abomination with a God Complex who has launched genocidal wars against innocent planets for no reason other than Fantastic Racism, For the Evulz or the people of those words refused to worship him as a god. He's also an extreme Omnicidal Maniac who takes sheer delight in the destruction it causes in the finale, and it's also the only RAPIST in Super Sentai, brainwashing Mahoro and making her give birth to Lijewl and was said to care nothing about the fact it's acts are despicable. This thing is the Sentai version of The First Evil, except we get to see it.
Agent Abrella, who'd sell many weapons supporting galaxy-destroying wars for fun and profit to most of the Alienizers, regardless of how many lives were taken. Once a galaxy is destroyed, he moves to the next to generate profit and destruction. Abrella treats any and all crime as a business opportunity, and it doesn't matter if you're destroying cities, selling slaves, pawning stolen merchandise, or filming child pornography, as long as the price is right, he'll be more than happy to sell you the tools you need to further your enterprise. And his Villainous Breakdown near the end of the series? All caused merely because the Dekarangers were ruining his transactions. Yes, Abrella himself caused a citywide destruction spree and nearly destroyed the whole of the Space Police just so he could keep his job.
Long the Immortal Dragon: So, in order to create the god of destruction and destroy the world, Long has to remove all worldly attachments from a person by systematically destroying everything they love. He did it four thousand years ago to Maku, causing the original rebellion, the Kensei had to use a forbidden technique and be turned into animals before he could destroy the world. Years later, Long destroys Jan and Rio's hometown. Jan loses his mother, forcing him to be raised in the jungle. Rio is taken in by Master Shafu, but eventually turns away and seeks evil power to get revenge on anyone. The reason Long did all these? He's just bored, and seeing that he's an ancient being, he thinks he has all the rights to manipulate these lesser creatures for his own amusement at their expense. There's no wonder that he's amongst Super Sentai most despicable monsters alongside Radiguet or Basco ta Jolokia.
If it's possible for a show that's relatively dark from the beginning to have a Knight of Cerebus, Sujigarano Akumaro would qualify as such. The Gedoushu at large wanted to create enough anguish to cause the Sanzu River to rise high enough to flood the world of the living, thus enabling their incursion thereof. Akumaro, on the other hand, wanted to open a portal to Hell because he wanted to enjoy absolute human suffering — even though said opening would destroy both the human world and the Sanzu River. He also made Fuwa Juzo's sword from Juzo's dead parents, whom he murdered himself. His monstrosity extends to even the Ayakashi who serve under him; they seem specifically made to cause as much suffering as possible in order to further his goal (using said suffering as a wedge into one of six precise points along a line, at the center of which was a portal to Hell).
Fuwa Juzo was a remorseless Blood Knight to the point that even his fellow Gedoushu didn't trust him; he even only became a Gedoushu in the first place so that he could keep killing in spite of his sickness. Akumaro saw potential in him for his ability to walk the line between humanity and the Gedoushu and sought to use him to open the gates of Hell. However, he throws a sharp wrench into the plan, declaring that he already knew that his sword made by Akumaro was made of his parents (a fact which Akumaro had recently told him to bring him to a Despair Event Horizon), and that he didn't care.
As early as 10 episodes in, Enter gleefully entered this position. Starting from his 'attempts' to appease his MajestyMessiah, which involves causing misery as much as he likes and pulling Unwilling Roboticisation, even on children, using them as human shields in much more frequently than Guirail could even muster and, on top of that, having a blast while doing them with an unabashedly relaxed, but still nasty speech pattern peppered with Frenchphrases. He's like a Basco that has been around ever since the first episode. Even after Messiah is apparently destroyed, he seems to escalate his horrors, going so far as to implant himself with two cards containing backups of Messiah's data—one of which also contains Masato Jin's data — in order to become the new Messiah, as well as implant Hiromu with another card to serve as his Soul Jar, forcing the Go-Busters to face the possibility of killing Hiromu in order to stop Messiah once and for all. He even threatens to harm Hiromu so grievously as to put him into a coma, just so that he can keep his Soul Jar alive without worrying about Hiromu attempting to off himself — assuming the card would let him do so.