Who is the most powerful Sith Lord?

Perhaps...

Darth Caedus

On all accounts Sidious was the most powerful Sith Lord of all. Don't be fooled by the simplicity of his "death" at the hands of Anakin Skywalker, Sidious had a mastery of the Force that went beyond that of any other Sith.
He had a mental grip on the galaxy which he used to control the weaker mind species and individuals. He hid his presence from the entire Jedi order as well as clouded there perceptions thru the Force. He straight up whooped Yoda and regardless of what some choose to believe dangle Windu from a string thru there fight solely to bait Anakin into joining him. He perceived future events at a higher level than Yoda. Kept himself alive by transferring his consciousness into clones. Put together the first, true, galaxy spanning Sith Empire and executed probably the most successful of all Jedi purges.
Then there's is a host of other small things that he's done that had dramatic results. For instance, fueling Darth Mauls hatred of Jedi by giving him dreams of raiding the temple. Creating various beast thru alchemy. Repeatedly killing and resurrecting the Death Star designer, leaving the memory of his death intact.
Sidious is the most powerful. Not just because Lucas says so, but, because Sidious has accomplished the most.