I would say that Plagueis was definitely the smartest in Bane's order. He lived a very long life and had learned many Force techniques that even Palpatine never learned. If you go under the premise that knowledge is power, then he was definitely the most powerful. However, he was far too trusting of his apprentice, and it cost him his life. Unfortunately, many of the tricks he knew were lost forever upon his death. Palpatine never learned all of Plagueis's techniques. I personally think that Palpatine was the most powerful.