More than one villain is one of my complaints with Trek sometimes. It seems we have 4 or 5 main villains and rarely do we get any new villain to deal with. But then we get episodes of the series without an actual villain so much as a situation to handle and they become brilliant. In TNG we had Moriarty become more brilliant than Data due to a programming screwup (someone who could outwit Data, not Sherlock Holmes) but he was less villain and more new life form and it was resolved so well. And in ST: Nemesis, we got a real villain who was original. Similarly, in WoK, we got Kahn (a returning character, but not a regular one) and in the reimagined ST, we got Nero who we could actually have some sympathy for considering what he witnessed happen.
Star Wars, though, is always the same basic villain, the Emperor and the Empire. Even in the prequels, it was all leading up to that point and Palpatine was *always* the villain, whether he was Senator, Chancellor, Emperor, or Darth Sidious, he was *the* villain of the SW franchise.