Darkonda. He was the one who kidnapped Karone in the first place and set her up to become Astronema, and on a few occasions, he forced Ecliptor to merge with him against his will. He even has no qualms about stabbing his friends in the back, as he was the one to kill Dark Specter—-all for trying to take over his position as Big Bad, and suffers a Karmic Death as a result.
Deviot. In a show full of villains with standards, Deviot had none. He was a manipulative, treacherous Starscream who would cheerfully throwaway his men's lives, thought brainwashing was a useful tool, and had no loyalty to anyone. Deviot destroyed Scorpius to get his hands on the cocoon that had been intended for Trakeena to awaken her true potential. He hurtles the Terra Venture into another dimension, and finally fights Trakeena for the cocoon, until they both fall in. His influence turned Trakeena into a Complete Monster too. Under said influence, she organized suicide bombers, killed her mentor for refusing to fire on civilians, and later tried to drain the lifeforce of innocent people to power herself up. In other words, it was him inside Trakeena's body and in full control.
Queen Bansheera; even in shows for adults, you'd be hard-pressed to find a worse boss. She has five main minions, and she's responsible for the deaths of four of them, and almost kills the last one, draining the life from one, blowing another to smithereens simply for being in the way, erasing the minds of the last two, Diabolico and Olympius (Her own son), forcing the Rangers to kill them, leaving only one, who remembered right before Bansheera killed him that he prepared an attack on the Aquabase, sparing his life. She's the only Power Rangers villain to get a Fate Worse than Death, as she ends up falling into the local Hell equivalent to be tortured for all eternity (or at least had her soul ripped out by demons. Oh, and the one who finally defeats her? The demon who performed a Heel-Face Turn, Diabolico, the only one who seemed to realize Bansheera was pure evil. Diabolico was the Big Bad at the beginning of the season. Bansheera was just a Bigger Bad at this point. She was that evil.
Master Org (both the Original Org and Dr. Viktor Adler): the original Master Org was an evil being that destroyed Animus, and when he was reborn, he destroyed Mandilok and kidnapped Shayla and took full control of Dr. Viktor Adler. Adler was once a decent scientist working with Cole Evans' parents Richard and Elizabeth. But the moment Richard got to propose to Elizabeth first AND make a breakthrough on their research, saying that he's jealous is a gigantic understatement. He proceeds to consume the remnants of the original Master Org, turning him into an Org himself, and takes great delight in killing Richard and Elizabeth (then-infant Cole himself only escaped his wrath because Adler couldn't find him). The original Master Org was more or less your run-of-the-mill Generic Doomsday Villain, but Adler was a human who was corrupted by his obsession with Elizabeth and the power of the Master Org.
One thing that must be considered is that Dr. Adler chose to be a monster. Most other villains were either born that way, had a Freudian Excuse, were raised to be evil, from a species who were Always Chaotic Evil, or otherwise had no choice over what they were. Dr. Adler is so monstrous because of the simple fact that he has no real excuse at all for being a monster (along with other villains in this page) like nearly every other Power Rangers foe. He became the second Master Org solely to enact a petty vengeance on two perfectly innocent people who had no idea they'd even incited his anger in the first place (not to mention a third, an infant, just for being the child of those two.)
Cole outright says only someone without a heart would do something like Adler did. Cole then checks (he is The Empath, and didn't want to fight Orgs at first until he determined via his powers that they were truly soulless.) and confirms Adler doesn't have one.
Mesogog. Despite having the sympathetic Anton Mercer, Mesogog is the evil alter ego who is determined to separate from Anton. He lied to Trent (White Ranger) when he made the trade for the Dino Gems when he said he would give Anton (who he had separated from) back, then he tries to kill them. To power his machine, he takes the power from his right hand man Elsa when she points out he needed a power source to give it a boost to start it. He Mind Rapes his henchmen when they fall behind or fail or just to keep them in line, and he uses Mind Rape on his own minions more than on his enemies. His master plan is to bring about a new Dino age. If you think about it that would mean many people would lose their lives. Through all of the Power Rangers series, Mesogog is played seriously. His being destroyed by all the power in the Dino Gems was most appropriate.
What makes Mesogog even more terrifying is that he is completely emotionless. He does all those atrocities and feels absolutely nothing.
At two points, he kidnaps a human and turns them into the Monster of the Week just to do his bidding.
In addition to the above, he shows the Not-So-Harmless Villain Lothor from the previous series, Ninja Storm he's Eviler than Thou. The main fight in the typical ranger crossover wasn't with the Rangers, it was between the two series Big Bads! Mesogog then imprisons Lothor and uses him for his plans.
Serrator. In "Broken Dreams", we find out he was the one who cursed Dayu and Deker, by tricking Dayu into becoming a Nighlok: he said that he would restore Deker in exchange of Dayu becoming a Nighlok, and after she accepts, Serrator reveals that he made Deker half-human and half-Nighlok. Dayu realized that Serrator tricked her and he laughed at her misery. This is what made Dayu a miserable and tormented person. Mia, after seeing this, feels sorry for Dayu and Deker. Also, unlike usual Deal with the Devil examples, it is Serrator who came to Dayu and probably did that when he heard Dayu's call for help and wanted to ruin her life and screw her and Deker's lives, and usually, in deals, The Devil tells what the person has to sacrifice or what he will do to that person, but Serrator didn't tell Dayu about what would happen to Deker. In his official debut episode, "Something Fishy" from Super Samurai, he destroyed a mooger army to show his power to Master Xandred. In "The Tengen Gate", he was referred to as "The Nighlok King". He did all of this in only small amounts of episodes. Serrator has only gotten worse since his introduction and has officially become The Starscream.
Serrator gets worse. It turns out that none of that was For the Evulz. He needed someone who knows the suffering of humans and Nighloks to break the barrier between worlds, which he considered more efficient than raising the Sanzu one Monster of the Week at a time. This is why he engineered Deker and Dayu's centuries of grief. In addition, his monsters are the ones who do the most horrible things to the populace. For one example, imagine if you felt like you were starving to death, but the more you ate the hungrier you got.
Switchbeast. To put that into perspective, he goes around town turning innocent people into various objects by trapping their souls into them. He later reveals that the point of this plan is that unsuspecting humans will get rid and destroy these objects, unaware that there are actually humans there. It is the point of his plan: to have a large number of innocent people (including Rangers, Bulk, and Spike) ''killed''. Usually, Nighloks want to flood the Sanzu River by causing misery, but Switchbeast wanted to do this by killing innocent people, and if Mike and Emily haven't stopped him, many innocent humans would die.
Doubletone, Monster of the Week from episode "Deal With a Nightlok", is seen as this by Kevin and Mia because he tricked Ryan into giving up baseball by lying that if he did so, his father would return from the army. He seemed to enjoy Ryan's suffering.
Master Xandred qualifies too. When he isn't drinking his "medicine" and yelling at people, he shows how evil he is, being the Big Bad and all. He treats his followers horribly, yelling at them, threatening to kill Octoroo at one point, attempts to kill Deker because he was meddling in his affairs, and his plan revolves around killing Jayden. Unlike Octoroo, who sends Nighloks to cause misery, Master Xandred summons an army of moogers to cause meaningless chaos in the human world and destroy everything. In episodes "Broken Dreams" and "Test of the Leader", we see that his moogers destroy entire buildings, meaning that he is responsible for hundreds of deaths of innocent humans. He brutally punishes Arachnitor when he tried to usurp him by mutating him into a mindless berserker slave to point at the Rangers and walk away. Granted Arachnitor was a traitor, but still. Even Octoroo says that his cruelty is boundless. In "Broken Dreams", he ruined Dayu's harmonium and attempted to erase her memory. She even says "I knew you were evil, but this..."
Eventually, after Deker meets his Final Death and Dayu's sorrow is so great that she is able to fully restore Xandred on her own, he rewards her by absorbing her so that her human side would make him immune to the sealing symbol. However, by this point, Dayu didn't care if she lived or died, and didn't resist.