Even Joker Admits One Gotham Villain Is His Equal 

One of Gotham's most hated criminal bosses, there aren't many villains  

The Joker might admire however, DC has proven that he is a fan of at the very least: The Riddler. 

Joker as well as Riddler are frequently enemies that are a part of Batman 

that have been caught in each others' hairs in the same way as The Batman's. 

Although they are often on the opposite side, the two have clashed on a number of occasions. 

One of the most famous instances is in The "War of Jokes and Riddles" episode of Tom King's Batman run.

The two criminals formed the alliance to fight The Caped Crusader throughout his second year as a police officer