Count Bleck is my favorite over the others because for several reasons: his dastardly personality, he has an original motive (all other villians wanted to conquer for the sake of power and control), and somewhat it is possible to relate to the character as you learn about his intentions and his past. He is also, in my opinion, the most powerful Mario RPG villain: he was capable of destroying the entire universe; all others wanted control of either the planet or the Mushroom Kingdom.