These are the villains from different universes that are not afraid to do what it takes to get their way and would make an unstoppable Suicide Squad. You would NOT want these four coming after you!
1. Malcolm Merlyn - DC Universe
He's incredibly charming, has connection to the League of Assassins (he just became the new Ra's al Ghul in "Arrow"), is great liar, skilled swordsman, and archer. He came back from the dead after supposedly being killed and freaking drugged his own DAUGHTER so she would kill someone for him! A guy with this much power who can also charm you and fool you into thinking he's the good guy would be a great asset to the Suicide Squad because he can be used to get on people's good side to get information out of them without them realizing it.
2. Nemesis - Icon Comics
The tag line used to describe Nemeses in teaser images of the comic were, "What if Batman was the Joker?" Now, just think about that. This guy has the wealth and technology that Bruce Wayne has but instead of using it to help people and bring justice to the world, he uses it to kill thousands of people FOR FUN. He has absolutely no remorse or or capacity to feel sympathy for anyone. He is WAY more evil than Lex Luthor. Amanda Waller would not have to worry at all about Nemesis hesitating to kill whomever she wants.
3. Joker - DC Universe
Even though it is unlikely the Joker would be on a team, he is one of the greatest comic book villains ever created, and would make a a great addition to the Squad if he was ever convinced to join. He messes with your head and isn't afraid to kill just to make a point or for entertainment which is a frightening combination. He can get his henchmen and Harley Quinn to help him if needed and and he'll also laugh in your face as he's beating you to death which is pretty evil if you ask me.
4. Voldemort - Harry Potter Series
He-Who-Must-Not-Be-Named is a freaking WIZARD who can kill you with one zap of his finger and have no qualms about doing it! He will kill anyone in order to get what he wants. He would be a powerful addition to the team and almost impossible to defeat (unless you're Harry Potter).