Just for fun, in no particular order.
Villains who are currently public domain, thus are free to use by anybody. This also includes Non-PEs who have plenty of PE incarnations. Also includes historical figures as well.
1. Dracula.
2. Nyarlathotep.
3. Satan
5. The Claw
7. Judge Holden (based on someone who may or may not been a real person)
8. Y'golonac
9. Jack Horner
10. App/Apophis
11. Adolf Hitler
12. King Richard III
13. Iago
14. The Witch (Hansel and Gretel)
15. Demiurge/Yaldabaoth
16. Mister Mind (specifically during his run in Fawciett Comics before DC bought the rights to Captain Marvel)
I'll keep expanding on this once more examples are brought up.