Who was the creator of the Star Wars empire

It really depends on what Sith Empire you're referring to, the Old Sith Empire was established around 6,900 BBY (7000~ years prior to the events of Episode I).
There was no 'creator' as such, it was formed by Dark Jedi who were exiled from the Galactic Republic after they were defeated at the Battle of Corbos, however they were lead by many ancient Sith Lords, the first being Marka Ragnos.
The Empire fell during the Great Hyperspace War during Naga Sadow's reign.





As for the Galactic Empire in the movies, it was established by Darth Sidious, however he was not a true blooded Sith.