Frieza or Kid Buu, they were both insane for no reason at all other than relishing killing.
Orochimaru actually has a really sad backstory that made him like he was; something about his parents dying when he was a tiny kid and then watching so many people die during the wars, he grew to hate death and wanted to become immortal so he and others wouldn't ever had to die. Eventually he just sort of lost track of his original goal and turned gradually insane until he became the monster he is today. Back before he turned traitor he wasn't that bad a guy, watching his flashback scenes with the other Sannin are really wrenching because he seemed so human back then.
Frieza, on the other hand, is a complete lunatic who just wants dominion and utter rule. He killed an entire race out of fear that they might defeat him based on a one-thousand year old legend that no one believed in. And he also blew up their planet. He's evil. Kid Buu's insane, but he has no concept of reality because he's just a creation of evil, so he doesn't even realise what he's doing is bad. He's still a close second, but Frieza wins because he knows exactly what he's doing and relishes it. Kid Buu just relishes it.