Emperor Revan
Most Powerful Sith Lord
I sure don't. Battle precog, having the Will power to resist the Dark side of an entire planet that no one else (not even Kreia) could resist, controlling the Star Forge, being perhaps the greatest tactician ever, having enormously high potential, learning everything about the Jedi from them, plundering tombs, relics, artifacts, and knowledge from a planet sized Sith storehouse of knowledge, learning dozens of ways to kill or turn Jedi/Sith, fighting for 6 years straight as the leader of one of the forces in the two different wars against powerful opponents, killing Mandalore and the strongest Echani, and considered to be the heart of the Force.
Then he loses all that power and becomes a Jedi again, he plunders Korriban, including the tombs of four very powerful dark lords, gaining their artifacts, learning from Tulak's holocron, killing two tarentateks at the same time by himself when just one could easily kill a Jedi, he killed everyone in a Sith academy that he started, he killed the best bounty hunter at that time, a Sith Lord, he killed hundreds of dark Jedi and assassin droids, killed his apprentice Darth Malak, and became even stronger than he was during his first reign (first paragraph).
Then he recovers all his memories and knowledge, increasing his power greatly from the second paragraph, before he goes to fight the ancient Sith empire single handedly.
I mighta missed something in there about him... but now we compare him to Tulak Hord:
Tulak is thought by Kreia to be the best duelist in a time of great duelists. Kreia also thinks Revan is power, and the heart of the Force.
So I'm going with Lord Revan.