Still Sidious, IMHO. He's ~ - slightly > Plagueis at this point, and I'm pretty sure I remember he already has a "greatest sith lord" quote by TPM. Seeing as how this comes after his training with Plagueis has been completed and he trained Maul in juyo, teras kasi and a bunch of other fighting arts, his technical skill with a blade likely does not increase drastically from TPM to RotS. In the Force, he's already powerful enough to be a match for "most powerful sith lord who ever lived" Plagueis, to shroud the entire Jedi Order, to knock the Force out of balance, etc. He and Plagueis have some pretty ridiculous speed and melee feats in the novel as well.