We all most accept that the only 2 Sith's that have ruled the galaxy by complete are Darth Sidious and Darth Krayth ... and I hate Darth Krayth ... so the most powerfull would be Sidious by his work conquering the galaxy
"Evil is a word used by the ignorant and the weak. The dark side is about survival. It's about unleashing your inner power. It glorifies the strength of the individual."