There are a few different interpretations of which Joker is the richest, so we will explore a few of the most popular options.
The first option is that the TV show version of the Joker, played by Cesar Romero, is the richest. This is because he was never actually shown to be impoverished and always had access to funds. It is possible that he inherited a large sum of money from his parents or had some other source of income.
The second option is that Heath Ledger's Joker from The Dark Knight is the richest. This is because he was shown to have amassed a large fortune through his criminal activities. He likely made a lot of money from robbing banks and other heists that he pulled off.
The third option is that Jared Leto's Joker from Suicide Squad is the richest. This is because he was shown to have a very lavish lifestyle, with expensive clothes and jewelry. It's possible that he made his money through similar means as Ledger's Joker did, but it's also possible that he inherited his wealth or came into it in another way.