Recurring villain Nina Myers (seasons 1-3). While terrorists have the excuse of being motivated by ideology, Nina is driven by her bottomless greed. Having infiltrated CTU, Nina fed information to the Drazen terrorists to assist in their schemes, and when Jack's wife Teri Bauer overhears Nina speaking German, Nina murders her to keep Teri from connecting her to Germany. Nina assists in numerous innocent deaths and terrorist activities, even conspiring to allow terrorists to get their hands on nerve gas. Jack Bauer once summed it up best: "You're worse than a traitor, Nina. You don't even have an ideology. You don't believe in anything."
Ramon Salazar from season 3 was a brutal psychopath of a drug dealer. Since his release from prison, Ramon killed numerous innocent people. Ramon was attempting to get his hands on a lethal bioweapon called the Cordilla virus for his own profit and when his own brother Hector caused problems for the deal, Ramon cold-bloodedly gunned Hector down, simply for a better payout for the Cordilla.
Navi Araz of season 4. His Big Bad boss Habib Marwan was ruthless, but Navi forcibly involved his innocent teenage son Behrooz in dangerous terrorist activities that took the lives of innocents, and then ordered Behrooz's girlfriend's death at Behrooz's own hand. When Behrooz proved unwilling, Navi attempted to murder him.
Abu Fayed has the most successful body count in Season 6. Fayed shows himself as nasty by having Jack Bauer tortured and later orders the same on a computer technician with a power drill. Fayed succeeds in having a nuclear bomb detonated in downtown LA, causing at least 13,000 deaths.
Jack's father, Philip Bauer, from season 6, was a Corrupt Corporate Executive who helped mastermind a conspiracy that reached all the way to the President, and helped have a previous president assassinated for it, with multiple innocents dying thanks to his actions. To cover his tracks he murders his co-conspirator younger son and later sells out his country upon being discovered, annoyed at the lack of appreciation for his work. Philip's work nearly started World War 3 and he kidnapped his grandson, fully willing to kill him if the boy didn't conform to Philip's standards of Philip's legacy.
Cheng Zhi has one of the best claims to be Jack Bauer'sArch-Enemy. Originally the head of security of the Chinese consulate, Cheng subjected Jack to horrible torture after capturing him at the end of Day 5. Originally a loyal soldier for China, Cheng began to think only of himself and his own advancement. Cheng subjected Jack's former girlfriend Audrey Raines to a near-lobotomy which left her damaged for a long time after. After being thought dead, Cheng resurfaces in Live Another Day, having been the one responsible for Margot Al-Harazi getting the override key for attack drones. Manipulating events (and eventually having a sniper kill Audrey), Cheng tried to launch a nuclear missile to begin World War III between China and America where millions would die out of petty vengeance for China abandoning him for his criminal actions.