Imperial side?

Agent. As others have said, it basically explains the last few hundred years of history (even pre-Revan) and how things were set up. It also explains why the True Sith Empire more or less collapses in Act III (you get hints of this in other Imp lines, but everything you need to see what happens with the True Sith Imperials is laid out before you). Indeed, you also see why Corellia goes the way it does, when the other three lines are not so clear cut.

Followed by BH or Sorc, and SW last.