I would have voted for Bane
Being a 'powerful' sith lord isnt all about shooting lightning and having a super costume. Bane is a revolutionary, The sith would not have survived were it not for the Rule of Two.
Plagueis seems to have been quite powerful, but it has never been verified that Palpy was telling the truth...Im also not sure what has happened to the Plageuis novel that was being discussed...
got killed way too early
. I think they could have developed it much more!! There's a sizeable vacuum in LOTF era EU now...