The Darth Revan story is quite outstanding. Revan was such a powerful and complex figure. His journey from a great Jedi to a Sith Lord and then back to a more ambiguous state was full of mystery. His influence on the SWTOR universe is huge, and his story has elements of redemption, power struggle, and the exploration of the Force that make it a standout among Sith stories.
I'd say the Sith Warrior's story is really good. As a Sith Warrior, you are in the thick of the action. You get to lead troops, engage in duels, and prove your worth as a Sith. There are story arcs that involve loyalty to the Empire, facing off against Jedi, and dealing with internal Sith politics. It's a story that showcases the martial and leadership aspects of being a Sith.