Despite being a member of what I consider to be the "Big Four" Bat-villains (the others being Joker, Penguin and Catwoman), The Riddler only made a handful of appearances on "Batman: The Animated Series." The creators have suggested that it was hard to come up with stories for him, which is a shame, because the character design and voice by John Glover could have made him one of the best baddies of the show.
So, for the Riddler's swan song, we have an episode that is in keeping with a theme from the later episodes of the series - reform and redemption. Of course, knowing the Riddler, it isn't genuine, even if Batman is the only one who believes it. The psychology of the Riddler - the master puzzler who cannot help but succumb to a challenge - makes him such a great foe for Batman, each smart and each obsessive in their own ways. This episode is also strong because it keeps us guessing. We know the Riddler is planning a return to crime, but like Batman we keep being disappointed when it turns out that Nygma is engaged in legal behavior. But when the payoff comes, it comes in a big way that certainly ranks as one of Batman's closest calls on the show.