Hmmm, categories.
Simple Bad Guys
No bad guy in Tolkien's works is just a 'simple' bad guy. Even Morgoth once strayed from the path. But if this constrains itself to the scope of LotR, then it's okay. These are the true symbols of evil.
Morgoth
Sauron
Witch King / Nazgűl (they surely used to belong into the Seduced category, but by the time of LotR and the way we meet them there, they belong to the Simple Bad Guys)
The Mouth of Sauron
The Ones Who Stray From the Path (in varying degrees)
This is a very broad category. Pretty much everything can be fitted into it. I will narrow it a bit for this reason: The ones who keep their good intentions (or think they do) but on the path to them go astray.
Saruman
Denethor
Boromir
The Seduced
They started out as good guys as well, but due to an external force that seduced them they turned to evil, but kept struggling.
Gríma
Gollum
The Exploited
These already started out from the not-so-good point, or at least had a weak spot that made them receptive. The evil seed that was inside them then was used by the real bad guys.
Ungoliant
Dunlendings
Dead Men of Dunharrow
Haradrim, Easterlings, Corsairs
The Created
Baddies that were created by bigger baddies to serve their purposes. Perhaps a sub-category of The Exploited.
Orcs
Werewolves and Wargs
Trolls
Natural Terrors
The baddies that act without a purpose, just out of themselves.
Old Man Willow
Shelob
Barrow Wights (probably started out as Exploited, while the old Angmar existed)
The Watcher in the Water
The Sackville-Bagginses
The Balrog (again someone who started out as Simple or Seduced, but in LotR he acts out of his own mind)
Smaug (the same)
Caradhras?
Where can we put Ferny or Saruman's thugs?