...going to be compiled of villains with of great power, but can be defeated, no hope for humanity, and has the ability to create absolute destruction. My list may change erratically as I am writing this, so their could be an assassination here or there. This won't be in the order of the best rated, it may happen coincidentally. Without any further adieu, here is my four.
1. Jean Grey: Dark Phoenix (Fox's X-Men series 1990's)
She is all the single dark female you'll need. Jean Grey's body was possessed by the Phoenix entity. The human host proved to be more trouble than the Phoenix had anticipated and soon turned to evil. Power, desires, and destruction became all she ever wanted. She is perfect for the squad because no one would try to stop her without her knowing, she can read your thoughts. But her evil teammates may be able to keep her freaky attitude in check, as they are as bad as she is.
2. Tetsuo Shima (Akira)
Part of a biker gang, teenager Tetsuo Shima had a head on collision with a child with special abilities. That encounter lead him to experimentation by the military transforming him into an angry gaudy killer. His resistance against apprehension lead to absolute terror, destruction, and a ton of bloodshed. He killed many people during his manifestation , rioters, police and military, disciples thinking he was their messiah Akira (he wasn't), he even killed one of his good friends and incidentally his girlfriend when he lost complete control. He is a sympathetic character, he was bullied as a youngster and he was seen as the bumbling little brother and not someone who mattered. Either way, he went evil and made the anime film Akira a total classic.
3. Agent Smith (The Matrix)
The main guardian or watchdog of the Matrix dream world the human race was trapped in. Mr. Smith was the ultimate antagonist and almost unbeatable in combat. He is was agitated with many things, seeing the human race as a disease and sick and tired of being confined to the matrix platform. What he didn't anticipate was the resistance of a Mr. Anderson (Neo). When Neo destroyed him, he manifested into a virus becoming even more monstrous dangerous villain. Even though the series as a whole was good looking but was lost in story and poor decision making. But Mr. Smith excelled in the series, increasing his hatred for Neo and humanity, act after act.
4. James Buchanan "Bucky" Barnes (The Winter Soldier)
He is the ultimate assassin, a soldier manipulated and enhanced to be a deadly killer. He, once a childhood friend of Steve Rogers (Captain America), became a more than worthy adversary of Captain America with no recollection of their relationship. In the Captain America Winter Soldier movie, he went on a killing frenzy that was precise and to the point, he almost couldn't be stooped. With a prosthetic metallic arm and a set of killer skills, he fits in nicely with the squad of high powered murdering psychopaths.
In closing, given there are so many great evil characters to choose from, sometimes you just have to settle. This team, yet very powerful, is not as nearly as unstoppable as they might think. With a death chip in their brains, they are just like any other flying monkeys when they are in control. But these are great flying monkeys. They all are emotionally compromised, all have aggressive personalities, and all will get the job done. Their souls are barely salvageable, but hey, everybody loves giving people a second chance. In their case, several chances.
Almost made the list:
Akuma Street Fighter II Turbo
The Juggernaut (X-Men)
Raoh The Conqueror (Fist of the North Star)
Meanwhile in the waiting room... Akuma and the Juggernaut butted heads, Akuma gives the Juggernaut a Shinryuken uppercut, while Raoh struck them repeatedly without them even seeing or knowing. They fell unconscious and Roah was asked to leave. ;-)