1. Darth Sidious - Stated by George Lucas to be the most powerful Sith Lord of all time, on top of the fact that he was able to defeat Yoda, and kill Mace Windu, Sidious obviously makes the number one spot. In the expanded universe, Sidious was capable of unimaginable feats such as creating hyperspace wormholes with the Force, capable of ripping apart entire starfleets, along with being a master of saber combat, being able to keep up with even the likes of Mace Windu. He also achieved immortality through the use of Soul transfer.
2. Marka Ragnos - The most powerful EU Sith Lord, stated in the summary of the Golden Age of the Sith comics, Marka Ragnos was an extremely powerful Sith Lord who ruled the galaxy for centuries, and his spirit lived for thousands of years after his death, and his return was even feared in the New Jedi Order, which existed 5000 years after his death. It was stated by Kyle Katarn that if Marka Ragnos ever returned, it would be the end of the New Jedi Order. He wielded a powerful Sith Sword, that could turn regular people into Force sensatives.
3. Lord Vitiate - Apprentice of Marka Ragnos, Lord Vitaite was capable of absorbing planets not only of all life, but also of all Force, ridding his home planet not only of life, but of the Force itself, making it impossible for anything to ever live there. Lord Vitiate was able to defeat even Darth Revan, and the Jedi Exile simultaniously, and possessed power over an entire Sith army, and was immortal, living for thousands of years in the Old Republic era.
4. Darth Vader (Pre-Injury) - The most powerful apprentice of Darth Sidious, at his most powerful. Before he was defeated at Mustifar by Obi-Wan Kenobi, Darth Vader was most definitely worthy of being within the top 5 Sith Lords, capable of defeating even the most famous Jedi Masters in seconds, and defeated Mace Windu in the Revenge of the Sith video game, who even Palpatine failed to defeat. Having the potential to become the most powerful Force User ever, and already being a swordmaster matching even Obi-Wan Kenobi, Darth Vader was one of the most powerful Sith Lords of all time.
5. Darth Bane (With Armor) - Creator of the Rule of two, Darth Bane was an extremely powerful Sith Lord that was powerful enough with the Force to pull moons out of orbit, and skilled enough with a lightsaber to defeat 5 Jedi Masters at once, along with his apprentice Darth Zannah. It was said that his Force lightning was powerful enough to instantly kill his opponents, and leave their corpses charred, and smoking. He wore nearly indestructible armor that even lightsabers couldn't penetrate, and was the Sith'Ari.
6. Darth Plagueis - Master of Darth Sidious, Darth Plagueis achived powers that no others have, ultimately resulting in the creation of Anakin Skywalker, and his own doom. Plagueis was capable of controling midichlorians, ultimately having control over life and death. He could create life, and sustain it within those who were dying. No other attained this power, and the Dark Lord's secret lay ever buried with him. Sidious was cautious enough to kill him in his sleep instead of fighting him one on one, showing that even the Dark Lord had those who could kill him.
7. Darth Vader/Exar Kun - Darth Vader (Mecha) was, though not as powerful as his prime self, still powerful with the Force, and skilled with a lightsaber. Still able to kill even the strongest and most famous of Jedi Masters easily, Darth Vader was the greatest Jedi hunter of all time, and was powerful enough with the Force to choke officers from miles away. He was capable of fighting evenly with Starkiller himself. Exar Kun was the most deadly practitioner of Niman, and wielded the first, and most advanced saberstaff of all time (Double bladed lightsaber), and his spirit lived 4000 years after his death, showing his true mastery of Sith Sorcery. Even while he was a ghost, he was able to affect the real world, and blow up pillars and boulders with blasts of pure Dark Side energy.
8. Darth Revan - Master of the StarForge, Darth Revan was capable of not only putting up a fight against Lord Vitiate, but also defeating Darth Malak easily, while he was combined with the power of several Jedi, and the Starforge, which is a feat that not many could achieve. Revan was a feared battle strategist, and master of an infinite fleet, but on top of that was a master of lightsaber combat, and had extreme exposure to both sides of the Force, he was even called the 'heart of the Force'.
9. Darth Nihilus - A Sith Lord capable of using Force Drain to drain all life from planets, and achieved immortality by binding his essence to his armor, making him more of a presence than a real man. Capable of killing millions at once, Darth Nihilus was easily more powerful than any other that lived during his time. Nihilus was capable of stripping even the strongest of Sith/Jedi of all of their power.
10. Darth Maul - Probably the best Sith warrior at being a Sith Warrior, Darth Maul, though powerful with the Force, prefered brutish combat, and pure saber battles, even risking his life to keep the battle sabers-only. In saber combat, he even bested Darth Vader in an EU Comic, though was bested in the end by Vader. Maul was easily one of the best swordsman of all time, and wasn't easy to kill, as he even survived being cut in half.
That's my top 10, who do YOU think are the top 10?