The Joker could be the most popular of Batman villains. He appears in two of the big screen Batman movies and is sometimes cast as the leader of the villains when they get together.
The Joker MInifig for the Batman LEGO theme is important to have if you want to challenge your Batman LEGO minifig to be the best (worst?) he can be. The Joker brings out Batman at his most resourceful, even if Batman has to stray from the path of hero.
Get yourself a Joker minifig.