Villains are important in any campaign. I believe, the villain create the hero. To any new GM, here are five villains you need to be able to generate on the fly. They are generally not the the ones who the PCs confront directly - not initially at least.
Emperor Palpatine (From the Star Wars movies) This villain is the one who has reached the pinnacle of power and now seeks to keep it. To many ambitious villain, the Emperor is a complete success story. He would rather seduce you to join him, but you refuse, he is quite ready to destroy you. Overconfidence and paranoia are the two common flaws this villain commonly has.
Jason Vorhees (Friday the 13th movie franchise)This villain is simply a psychopathic madman with a singleminded focus and no intention to share or work with others. He proceeds on his path destroying anyone he encounters. This type of villain usually makes a lot of enemies, creating a case for the "enemy of my enemy".
Magneto (Marvel comics super villain) This is the villain with a clear idea and a definite, easy to state goal. As such, he can be almost predictable in his action, but therein also lies his power. Although his ultimate means may be villainous, many heroes might share some -or all- of his ultimate goal.
Igor (Classic Horror movies) Named for the usually deformed villain's henchmen of classic horror films, this villain is one who passes himself off as a or as a weakling to deflect all danger away from him. As such, he lets others take credit for his work as he secretly amasses the power he needs. Chancellor Palpatine (from Episode 1 and 2) fits this bill. The Igor is not in a hurry, and is willing to make temporary alliances as long as he benefits in the long run.
Hannibal Lecter (From the Hannibal, Red Dragon, Silence of the Lambs movies) This is a villain who is completely rotten and insane, almost as though one chose that path of evil, and is perfectly happy with how he is. The Hannibal Lecter can be flattered into helping or pushed off the rail to become a Jason Vorhees.