The literal answer is "whomever The Force currently favors to work it's will". Understand that "The Force" isn't some personal form of mysticism people master entirely due to their own abillity. It's a sort of governing entity that guides the universe through cycles with good and evil each having their periods of "control" with a period of balance in between. "Star Wars" is the story of an age of good coming to an end to be replaced by an age of evil. This is why the prophecy (sent by The Force) is that Anikin will bring balance to The Force, for the good guys who were in control the only place for them to go was down. Darth Sidious made the mistake of thinking that he was going to be the ruling force of the new age of evil, he was not, he was a tool to bring about the transition point. Vader achieves balance by killing all the Jedi, and then killing The Emperor. Luke being sort of a tool to get him to do both.
In an age of evil, whomever rules The Sith is probably the most powerful guy around. In an age of good, it would be whomever leads the Jedi. In an age of balance, neither side has any real advantage and probably doesn't have anyone on that power level.
At the time of "The Old Republic" games, I'd imagine it's fundementally an age of evil coming to an end, albiet the majority of the evil was located out there in the Sith empire. The big guns for The Sith would logically be feeling the force becoming "cloudy" (as many of the Jedi put it) and their powers reducing accordingly. The end result of this period is that The Sith and their empire are going to be wiped out so totally that come the end of the next cycle (in the Star Wars movies) nobody will know for sure what a Sith even was.