1. Darth Revan and Exar Kun
its way too hard to distinguish between these two. both were masters with the blade and the force. they were both easily the most powerful of their time as well. Revan may be smarter and possess more knowledge than Exar. He was able to see how both sides of the force were used and he saw that both sides of the force had flaws and good things about them. revan was also able to effectively use techniques and teachings from both sides to his advantage. and most of all, he was neither the jeedai nor sith as heroes or villains, just different sides of the force, something Luke would later realise as well. but exar had unbelievable skill and force mastery and in an all out duel, id say exar would be better with a blade.
2. Darth Traya
A lot of people underestimate Traya. I mean i dont think ive seen her mentioned in this thread yet, although i may be mistaken on that. Traya was almost as powerful in the force as revan IMO. she was able to use echoes in the force to instantly kill a force user with a wave of her hand, as seen in the light side KOTOR II. she was also able to use telekenisis to give lightsabers almost free will, rather than being restricted to a certain lightsaber form. And lets not forget her cleverness which i believe surpasses sidious. i mean she was able to use the exile to kill the jeedai council and both sith lords. she lied to the exile in the beginning of the game and told him that it was the jeedai council who took away his connection to the force, knowing full well that he would seek out the council members and allow her to kill them. she also told Tobin about the jeedai academy on telos, which caused nihilus to attack the planet and show himself to the exile. she eliminated all powerful force users in the galaxy, svae for sion, revan and the exile, and took the power for herself. she would have reigned supreme as well, if the exile hadnt ruined her plans.
3. Darth Malak
Another sith lord who a lot of people underestimate. True Malak was owned on the star forge, but people forget that it was revan who owned him. Malak was able to almost accomplish what traya and sidious did, wiping out all the jeedai. while malak was a pussy, he was a powerful one you must admit. he used the force to get the star forge working at 200% of its potential. he killed two jeedai knights as though it was nothing, and would have won the jeedai civil war if it werent for revan. malak also found a way to keep jeedai alive and absorb their life force through the force, making himself even more powerful.
4. It was a tough one, but in the end ill have to say Ulic Qel-Droma. Exar Kun saw him as the only Sith in the galaxy who could challenge his might. that has to count for something. He also battled Exar to a draw before he became his apprentice, which would be no easy task. Ulic also defeated Mandalore and gained the support of the Mandalorians in the Great Sith War. and after he was cut off from the force he was able to defeat a jeedai, which once again is no easy task.
those are my five. keep in mind i didnt include any of the ancient sith, mainly ragnos, sadow and freedon nadd. i just dont know enough about them to form a good opinion about them. i also didnt include nihilus cause hes a cheap son of a *****