The List
1. Darth Sidious
2. Darth Bane
3. Darth Plagueis
4. Darth Caedus
5. Lord Vitiate
6. Darth Vader
7. Darth Zannah
8. Darth Malgus
9. Exar Kun
10. Darth Tyranus
11. Darth Krayt
12. Darth Maul
13. Darth Marr
14. Darth Nox
15. The Wrath
Honorable mentions
these are people who should be in the top 15 somewhere, but are too difficult to place.
Darth Tenebrous- IMO he's right below ROTJ Vader
Darth Cognus- She's probably about Zannah level
Tulak Hord- He's probably between Nox and Vitiate
Freedon Nadd- He's probably about Vader level
Marka Ragnos- He's probably about Vader level
Darth Nihilus- IDFK
Kas'im- He's probably between Wrath and Maul