I would choose Morgoth(Melkor), simply because he created hate itself. And also once commanded an army of thousands of orcs, evil men, dragons, Balrogs and demi-gods(Sauron was one). It took all the Valar(Gods) to take him down.
But since he's not an option, I'll go for Sauron.
Reasons:
1.He basically became the concept for Dark Lords in every fantasy/sci fi franchise.
2.Killed both the King of the Noldor, Gil Galad, and the King of Arnor and Gondor, Elendil.
3.Has a badass suit of armour.
4.Has a mace that can K.O. dozens of warriors at once.
5.Using the language he created causes fear in the hearts of those who hear it.
6.His lieutenants are immortal, and are themselves badass.
That's just my point of view. Others may differ.
But since he's not an option, I'll go for Sauron.
Reasons:
1.He basically became the concept for Dark Lords in every fantasy/sci fi franchise.
2.Killed both the King of the Noldor, Gil Galad, and the King of Arnor and Gondor, Elendil.
3.Has a badass suit of armour.
4.Has a mace that can K.O. dozens of warriors at once.
5.Using the language he created causes fear in the hearts of those who hear it.
6.His lieutenants are immortal, and are themselves badass.
That's just my point of view. Others may differ.