If I had to say anyone was the "worst villain", it'd probably have to be Dane Vogel from Saints Row 2.
Dane's a big-shot for the Ultor megacorporation and he treats the city Saint's Row 1 and 2 take place in like his own personal playground. He manipulates people into thinking he's the innocent party in everything that happens regarding Ultor or that they have no standing in the gang wars (referenced from the Brotherhood chapter). His motivation for doing what he does is nothing short of "because I have the money". The last story mission in the game is kill his ass, a mission I was overjoyed to do because in the last part of the mission (warning, spoilers) you shoot him in the face and he falls maybe 100 stories to the ground. Then you escape by helicopter to see your buddy shooting cops with an assault rifle.
Dane isn't any real villain, he's not doing this for money or power, he's already got that. If he did have any sort of motivation for being the villain it's because he drew the short straw from among the three gang leaders (Kazuo Akuji, The General and Maero), himself and Troy Bradshaw.