Marvel or DC doesn't matter which. Who's your favorite villain and what story with them in it was your favorite?
This question has been asked time and time again, and I guarantee you that if you go look at every poll that's ever been made concerning that topic, 2/3 will say the answer is Joker.
The 1/3 is always Magneto, with Doom following at 3rd.
Kang the Conqueror is a time travelling monarch from a future earth that has conquered thousands of worlds in his own time and many different time lines. One of his only failures is modern day earth with the Marvel heroes. He is a ruthless conqueror but chooses to fight honorably in battle as opposed to killing people when they sleep or manipulating the time stream as his future counterpart, Immortus, likes to do. Kang hates the fact that he becomes Immortus and fights against that eventuality. He is involved in two of my favorite Marvel story lines, both of which written by Kurt Busiek. In Kang Dynasty, he succeeds in conquering the earth for a few months before the heroes are able to rise and take him out. There are some awesome fights between Kang and Captain America. He is also in Avengers Forever in which Avengers from different time periods are pulled together for a story that spans much of Marvel history. It is an awesome story that has great moments for Kang. He is a character like Doom that doesn't always fight for evil. He fights for himself and sometimes that lands him on the side of the angels.
He is also the main antagonist on the last arc of Uncanny Avengers. The Avengers: EMH cartoon did a pretty good job portraying him in their several part episodes around him.
And nobody looks cooler sitting on a purple, floating beanbag chair. (I am trying to upload a picture but having some technical issues).