Originally Posted by ThePhantasm
Unfortunately there are some rather lame episodes. But The Laughing Fish is incredible, really creepy and arguably the best Joker episode of the show. Also a must see is Mad Love.
Then, in Justice League, there's the two part Wild Card. The first part is rather tame, but the second part, especially the finale, is spectacular.
Those three stories really get the Joker character right in my opinion. He's scary, disturbing, funny, and a real threat.
I was gonna say this.
As for the villians, I would only repeat the other options already mentioned: The Riddler, Mr. Freeze, The Scarecrow, The Penguin and Poison Ivy, in that order. Bring those five in a fresh compelling way.