The Dragon Ball franchise is the Trope Codifier for modern shounen action series, including its villains. Most of them are Card Carrying Villains who do things out of the heck of it, but not too evil. And there are some who were created evil and do some questionable things. Then there are the few who are so twisted that they are memorable for being so ungodly demented and cruel and lacking the morals/mercy of their fellow villains, who are either horrified and can't do anything about it or actually admire the person due to their overwhelming power.
Red Ribbon Army Arc: Tao Pai Pai is an assassin who cares only about profit and ruthlessly and remorselessly wipes out anyone who gets in his way with his bare hands. After he murders Upa's father and defeats Goku in battle, he laughs at Upa and tells him that he's lucky that he's still alive, meaning that he has absolutely no problem killing children. One of his worst offenses has got to be when he forced a tailor to overwork on making an outfit for him in about three days...which he rewarded by killing the tailor. When he returns to Karin's Tower to look for the one Dragon Ball he neglected to find, he grabs Upa and throws him against the tower (fortunately, he was saved by Goku). After his defeat, he was rebuilt into an Ax-Crazy cyborg who wanted nothing more than the deaths of both Goku and Tenshinhan. He's a major Knight of Cerebus for the series.
Namek Arc: Frieza has more than earned the mantle of being the most evil villain of Dragon Ball Z.He was responsible for the extermination of the Saiyans, despite their loyalty, since he feared he was going to be overthrown by a Super Saiyan. During his invasion of Namek, he killed many civilians, including Dende's caretakers and brother, even after he got the Dragon Ball. He also slaughtered several warriors and tortured Nail, Gohan, and Vegeta to near-death, and had Vegeta crying and begging Goku to stop Frieza, who then shoots Vegeta through the heart. He then blew up Dende, when he learned the boy could heal people. He finally blew a hole through Piccolo, proceeded to blow up Krillin, and threatened Goku's son, which led to Goku becoming the very thing that he feared most. If Shin Budokai is anything to go by, he was also directly responsible for driving Broly Ax-Crazy when he blew up Planet Vegeta without even realizing it, and he freely admitted that all the horrible things he did in stories were indeed true. What makes him so horrible is that, all of it was done willingly. He wasn't born to be a monster like Cell or Majin Buu, or insane like Broly and some of the other movie villains, at least, not at first glance. He just reveled in his power and despite having the funds to live a life of permanent luxury, instead founded a company whose business was wiping out populated worlds to sell to the highest bidder.
Android Arc: Cell, an Ultimate Lifeform created by Dr. Gero, inherited Frieza's sociopathicsadism and Vegeta's pride. He goes around towns eating thousands of people with his tail just to increase his powers with the last being disturbingly prolonged in order to give the latter absolute fear of being absorbed, then absorbs Androids 17 and 18 which results in his going One-Winged Angel. Though programmed to kill, he has a sadistic pleasure in it that is all his own doing when he starts "the Cell Games", just to hold off destroying Earth so that he can see everyone's faces contort in terror and states that he would've hunted every single person on Earth one by one if he won. He beats Gohan to near death and creates "Cell Juniors" to torture the other Z Fighters so that he can transform to his Superpowered Evil Side, which he succeeds by destroying Android 16 in front of him. When the fight stopped going his way, he tried to destroy the world with his self-destruct sequence, forcing Goku to perform a Heroic Sacrifice.
Babidi orchestrated the revival of Majin Buu, ultimately making him the one responsible for the cataclysmic amounts of death and destruction that occur later, and killing his men when they are no longer useful. He kills off brainwashed minions Spopovich and Yamu commenting how enjoyable their deaths were, and throws Dabura, his loyal right-hand man, under the bus once Buu is revived. He took great delight in having Buu systematically kill the people of Earth so he could lure out Piccolo, Goten, and Trunks all for the purpose of getting his petty revenge against them. When he's asked to stop this by Goku, he then announces that he'll have more people killed just to spite him. Finally, he the head of someone who gave him the names of Piccolo, Goten, and Trunks, just because he found the information to be useless. He also constantly threatens Majin Buu when the latter doesn't obey him, which later results in Majin Buu giving Babidi his karmic death. He's up there with Frieza in terms of just being an evil asshole that doesn't have the excuse of being a creation born evil or insane.
Super Buu has the evil of Kid Buu but, unlike Fat Buu, who is too childish to know better, has the ability to understand about his own evil and not care. This is a guy who, once made eloquently intelligent by Piccolo, actually lamented killing off all of humanity because destroying their cities wasn't as fun without them screaming. When told by Piccolo to go kill off some humans to buy some time for Goten and Trunks to master the fusion technique, his response, out of impatience, is to fire a barrage of energy that locks on and kills every human outside of Kami Tower except Hercule; this is conveniently called Human Extinction Attack, because it was fire as many blasts needed to bring the human race to extinction . He also turns Chichi into an egg to smash it because she was annoying him. And once he's fed up, he turns Krillin and everyone except Gotenks and Piccolo into candy and eats them alive. Finally, once he does have Piccolo's intelligence, despite being superior to a Gohan who had trounced him earlier, he beats him senseless while using Piccolo's teachings to mess with his head.