Darth Bane was probably the most talented force user of his generation.
Being fresh from the book Dynasty of Evil (I still have it on my desk actually waiting to be returned to the library), Bane considered Zannah to be stronger than him at using the force. She did also defeat him with her force powers. Her mind was stronger than his too which was why he failed to take over her body.
Generally Bane was considered stronger at dueling, even in his weakened state, while she was considered stronger with force powers, even by Bane himself.
Unless you consider Zannah to be the next generation, then I'll agree with you.
Overall though you're still right in regards to Sidious.
However what we also have to consider is whether the Jedi order gets stronger in comparison to the Sith. They don't have such a rule as the rule of two.
Also while the rule of two is in place to keep the Sith strong it's not always the case that the next Sith may beat the previous Sith at their strongest. Zannah certainly didn't beat Bane at his strongest, his body was greatly diminished because of the Obelisks, and arguably probably wouldn't have in a lightsabre duel. She could only just match him even as he was weakened.
Another point to take into consideration I guess.