1. Thanos: I mean why not have Marvel's most badass villain who can easily wipe out any foe.
2. Darkseid: I personally think Darkseid is DC's best villain for a number of reasons. He cans shoot laser from his eyes that will follow you, he can shoot fire out of his hands and channel dark energy. He almost single handedly took out the justice league.
3. Black Adam: this guy would send a chill down anyone's spine.
4. Ra's Al ghul: Intellect and ability makes him the Batman of this Suicide Squad