The Joker costume has become a very popular costume ever since the release of Christopher Nolan's 2008 film, The Dark Knight, though it was also popular for a time after Tim Burton's 1989 film, Batman. Like his arch-rival, Batman, the costume can come in many versions based on the various appearance of the character, from the different comic book ages, to the original 1960s show, but most popularly, either Jack Nicholson's or Heath Ledger's variant. His appearance in the Arkham video game series can also be a popular choice.
The character is the perfect psychopath, and his evolution since his creation can allow the costume wearer to play the character in either a wacky or extremely creepy sort of way.
The character can be paired with several other characters as a perfect match. Not only can the character be partnered alongside a different version of the character (this works especially well when done with a Nicholson/Ledger variant duo), his lover, Harley Quinn, or his arch-rival, Batman.