Palpatine was a Sith Lord, hence the title Darth Sidious.. but once he became Emperor he couldn't use such a title, nor did he need it. Controlling Anakin/Vader is another form of power, one that he used well.
As I said before, Palpatine was if you look at it from a different perspective, the most powerful overall. This is proven because he conquered the entire galaxy, just because he did it through manipulation, control, etc doesn't mean he isn't powerful. It is merely a different type of power. Also don't doubt his force abilities and power until you see ROTS, because right now all we have to rely on (with physical power) is the end of ROTJ.