Sidius was the strongest in the way that he was the smartest and most clever one.
If both had existed at the same time, im certain that sidious would have had a very devious (and long) plot to overthrow the emperor.
Maby he would even have taken over the republic and then recruited siths into fighting the sith who were loyal to the emperor?
who knows, but either way, sidius was the strongest overall.
But the emperor is definately stronger in the use of the force.
How so? He uses trinkets to accomplish his feats whereas Sidious was capable of destroying planets with his power alone. How is the Emperor stronger in the forcE?
As for Butchex.
Being immortal doesn't make one stronger. Set Harth managed to live for 400 years but he was never particularly powerful. Quite the contrary. He was very "mediocre." in terms of power.
Don't forget that Sidious was much much more powerful than Yoda who lived to be 900 years old.