I would say Darth Andeddu is one of the most powerful to. He was one of the first to use the "Darth" title. Also Darth krayt is pretty powerful.
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Eh Darth Krayt and most of the newer Sith aren't as powerful as the older Sith.
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Krayt took over most of the galaxy though. But Andeddu was one of the older ones from thousands of years BBY. His holocron was found by Bane and later, Tyranus.
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Krayt took over most of the Galaxy because he was a good leader. That doesn't make him powerful as a Sith.