There are some fantastic villains (and even heroes) that could make the Suicide Squad. Some could make it more powerful than the Justice League, even. But, in the spirit of the comics squad, I'll be keeping the power levels down. Each one of my picks will be mid-tier power at highest, and all will rely on skill first, powers second.
1. Taskmaster (Marvel Comics)
Marvel's Taskmaster may be the best suited Suicide Squad leader in comics-including DC. Not only is he a proven leader, his abilities make him one of the most skilled combatants in the Marvel universe. He can remember and copy any fighting styles he sees almost instantaneously, and never makes the same mistake twice. He's an expert with any form of weaponry, and one of the best marksmen out there. If anyone is gonna lead Task Force X, it's gotta be him.
2. General Grievous (Star Wars)
The four lightsabers say it all- not only is General Grievous a strong, fast, and agile cyborg, he's killed some of the galaxy's most formidable warriors (23 named, by some counts). All of this is without the Jedi's force sensitivity. Bullets aren't gonna do much against this guy, and with his spinning blade technique, he could turn any enclosed area into a killzone.
3. Bellatrix Lestrange (Harry Potter)
Bellatrix really would have fit in at Arkham-she was as insane as they come. Responsible for more tortures and murders than almost any other Death Eater, she was also one of the most powerful witches under Voldemort's command. Her wide array of powers would come in handy not only for combat, but entry and exfiltration through whatever compound the squad is assaulting.
4. Khan Noonien Singh (Star Trek)
Khan perfectly rounds out the team. As a genius beyond any human capacity, he would be invaluable for using, defeating, or hacking through various advanced tech the others couldn't handle. He's also a highly skilled marksman and hand to hand combatant, and has greater than peak human strength, speed, agility, durability, and endurance. He's formidable in every way, and fills the tech gap in the squad.