Piedmon is the leader and the most powerful of the Dark Masters, leading them in their conquest of the Digital World. A psychopathicMonster Clown, he enjoys toying with his victims before finishing them, in contrast to his more straightforward associates. Reducing his territory to a barren wasteland, he monitors the actions of his fellow Dark Masters and the main cast, allowing his allies to die without ever interfering. He participates in the murder of Pixiemon with the rest of the Dark Masters, kills Chumon himself, nearly kills WarGreymon, then transforms most of the Digi Destined and their Digimon into keychains, which he toys with as he hunts the two youngest members of the team down, intent on keeping them all trapped as toys, forever. He's eventually revealed to be The Man Behind the Man to every major villain in the show, from Devimon through fellow Complete Monster Myotismon, and their actions and those of the other Dark Masters can all be laid at his pointy-shoed feet.
Professor Akihiro Kurata is the most despicable human villain to ever appear in the Digimon franchise. While Ken from Adventure 02 didn't think that he was doing anything wrong (and may or may not have been under More Than Mind Control), Oikawa was under Mind Control in a significant way, and Yamaki from Tamers was a Well-Intentioned Extremist who ultimately had a Heel-Face Turn, Kurata's only goal is to Take Over the World and sadistically destroy all Digimon because of a petty grudge. Creating the Gizumon, mechanical monstrosities that make other Digimon Deader than Dead by stopping them from reincarnating like usual, and experimenting in creating half-Digimon hybrids were only the start. Creating a weapon of mass destruction to kill thousands of Digimon in one shot, forcing the Teen Genius on the heroes' side to a Face-Heel Turn by threatening to kill his Ill Girl little sister, trying to replicate his experiments on said Ill Girl, and finally awakening a mighty Demon Lord and merging with it to destroy all of his enemies...well, that should give you an idea of how loathsome this guy is.
Laughing hysterically as Ikuto/Keenan sobs over the loss of his surrogate father, whom his Gizumon have just permanently deleted. He is not just the most despicable human villain to ever appear in the Digimon franchise. He is the most despicable VILLAIN to ever appear in the franchise. Period. He even beat out Myotismon, the previous holder of the most complete monster in the series - not an easy feat.
A very good showing of just how horrid this guy is would be the reaction the heroes have to him in the last phase of the final battle. Marcus finally gets ShineGreymon to reach Burst Mode and turns the tables on him. Cue ShineGreymon shooting Kurata one of the most hate-filled Death Glares in anime history while everyone else cheers for Kurata's demise before Marcus and ShineGreymon put Kurata down while he's begging for mercy. Normally, trying to kill someone off who's begging for mercy is the job of the Anti-Hero or in a Big Damn Villains moment, and if the hero does it, it's normally a sign that they've crossed the line. Kurata is such a horrible, evil, terrible excuse for a human being that finishing him off while he's begging for mercy is a completely acceptable action. Kurata is the digimon equivalent of Doctor Weil.
Tactimon bears special mention in that he was prepared to torture a child to death, as opposed to a quick, painless execution (not that that's any better; it just shows how sadistic Tactimon is). As if that wasn't bad enough, he's a full follower of the We Have Reserves mantra and has no compunctions about sacrificing his own subordinates for the sake of maintaining his supposedly perfect battle record.
Another example is NeoVamdemon, an Expy of Adventure's Myotismon, who absorbs a great number of innocent Lopmon in order to remain immortal, while also absorbing Kiriha's partner, MetalGreymon, into himself. Then he basically forced them to destroy themselves in order to avoid letting him use them as batteries anymore.
Gravimon, considering how mercilessly he Mind Raped Kiriha and set in motion the events that resulted in, among other things, the death of Deckerdramon.
An underrated but important example is Millenniummon of the Digimon Wonder Swan Series. A reborn Machinedramon, he is responsible for the cataclysmic events of the Digimon Adventure/Zero Two by traveling through time and releasing Apocalymon. In Digimon Wonder Swan Series, it is stated that Apocalymon's existence brought Devimon, Myotismon, and Dark Masters into existence. By releasing Apocalymon, Millenniumon is responsible for everything that goes wrongin Adventure and Adventure 02 and is responsible for the creation for some of complete mosnters in this page. His Zeed incarnation is even worse; by Brave Tamer he's actually destroyed almost everything in his timeline and turned it into a Crapsack World with himself as its king. And this isn't even taking into account the Inferred Holocaust that must have happened when he broke the Digital World in half.
When Milleniummon was destroyed, his data turned into Dark Spores and one buried itself into Ken's neck, exerting its influence on him. Since then, it grew within Ken and was later harvested and implanted into other children by Yukio Oikawa, meaning that Milleniummon is so evil that his data can corrupt anyone and turn them into insane and hateful beings.