1. Darth Revan, if you want reasons read Human Vader's last post
2. Exar Kun, I believe he's weaker than Revan and Revan's much smarter and level headed so that's why Kun is second.
3. Darth Traya, again see Human Vader's last post if you want reasons. Except I don't think she ever wanted to rule the galaxy, I think she did all of that to train the Exile for some reason, perhaps to fight Lord Revan.
4. Emperor Palpatine. Competes with Yoda, killed 3 Jedi masters in about 10 seconds with his lightsaber, etc.
5. Darth Malak (once again, see Human Vader's post.
I (again like Human Vader, I didn't know we were so much alike!) don't include anyone we know too little of. For instance Ragnos, Sadow, Hord, Freedon Naad, and Nihilus because we don't know much at all of his draining ability.