It’s interesting that for all the camp humor of Batman’66, Frank Gorshin’s Riddler might be the most malevolent Riddler of the 20th century, and still one of the most malevolent versions of the character to date. There are few interpretations of the Riddler where the concept of Batman’s death brings out such sheer, manic glee bordering on the nearly orgasmic.
It is that sense of intensity that makes Gorshin’s portrayal still the best, IMO of all versions of the character either live action or animated. John Austin was fun, and Jim Carrey had a comedic element but neither came close to the true sense of villainy that Gorshin brought to the role.
I think it really shows the Riddlers’s ability to be just as strong a Batman villain as the Joker, if given the opportunity.