I tend to connect most with Revan due to the fact that his power derives purely from his genius, and I've always tended to appreciate villians displaying ungodly amounts of intelligence.
Let's face it, strictly from everything I've read, Revan is praised more than any other Sith for being the single greatest military strategist.
If I were to answer this question based on raw power alone, then the obvious answer is Nihilus because his power is only limited on his source of "food".
However, Nihilus's greatest advantage is also his greatest weakness. His hunger was the downfall of him, and seemed to drive him right into the hands of those who would kill him.
Sidious never impressed me in the slightest. Sure he was powerful, but no where near as powerful as Kun or Nihilus, and he was nowhere near as intelligent as Revan. He was a trickster, and his deceit was really his greatest weapon, as he used it to pull the strings of the republic long enough until he could convert Anakin and build his droid army and his death star. Anakin was more powerful than him and he knew it. He had little or no military strategy, just an extremely large and powerful army. His power only came from pure opportunity and convenience.
Kun was the most well-rounded. Not AS powerful as Nihilus or as intelligent as Revan, but excelling in both traits. I suppose this would make him the most powerful sith lord ever since it takes both brains and brawn equally. I mean hell, the dude still killed people many millenia after his death and wasn't ever fully destroyed until the book I, Jedi.
I still like Revan the best though, especially because when I play Dark Side on KOTOR, I MAKE him the most powerful ever =P I flick people to death and laugh.