For an unhinged homicidal monster, the Joker has a surprisingly varied dating life.
While you'd presume the opposite to be true - half because Harley is so iconically his love interest, and half because it seems like his obsession with Batman would get in the way of pursuing anyone romantically - there's a fair collection of comics that show just how much the Joker has gotten around in his time on the page.
Between dead wives, Empresses, and even Atlanteans, Joker has wooed more characters than any person with a shrine to the Dark Knight rightfully deserves to be able to romance. While these relationships literally never end well, it's clear that the Clown Prince of Crime is uncomfortably good at making people fall in love with him - even when he's completely open about how little he cares for whatever poor unfortunate has fallen for him. Combine this with several completely surreal alternate universes, and the Joker's list of exes becomes something that has to be seen to be believed.
Taking the old cliche that 'girls love bad boys' to it's logical, nightmarish conclusion, here's the Joker's entire sordid romantic history - for better, or for worse.