If you're going to put anyone on the list, put Exar Kun. Easily in the top 5 most powerful Sith. Makes a major play for number 4 or even 3. I'd also put Odan-Urr, but he's more of a Historian.
Edit: Now I would put Odan-Urr on my list, but that's more of a personal preference. I don't know where he sits exactly.
Exar Kun is an incredibly interesting sith to me, i don't know why, his story is the usual Sith gets destroyed by his own allies, but i still find it interesting...
Ill have to look more into his actual power before i do anything though, i know he was the greatest lightsaber duelist of his time, but thats about all i know of his actual power levels.