Joker has a great visual design that is simple yet elegant, but he isn't really that interesting of a character. He's pretty much a foil to Batman, an obstacle, his motivations are lazy and shifty, his intelligence is loosely defined and poorly convenient to the plot and his relationship with other characters is mostly basic antagonism. He strives to be the symbol of an idea, but unlike Superman it fails at having room for development. He just is. Think of a Street level Galactus.
Magneto and Doctor Doom are more interesting for a number of reasons, both are terribly flawed and entertain complex relationships with their enemies and rivals. The have shades of gray so for the most part you don't want to see them go too far, they set an expectation and it paces their fiction, a story where they appear rapidly becomes part of their overarching plots.
