My favorite Batman '66 villains:
Newmar's Catwoman in her early appearances. By the time she was done with the part, the character had regressed from a sizzling femme fatale into a confused girly-girl who seemed more interested in trying to romancing Batman than getting away with the theft of some valuable bauble.
Gorshin's Riddler. As mentioned above, he was able to balance comedy with sincere menace. The very first episode of the series is a good example of this; the first cliffhanger is also rare because it's one of only two episodes in the entire series where someone actually dies.
George Sanders as Mr. Freeze. There's a great moment where he confronts Batman in Freeze's sub-zero lair and he torments Batman by turning off Batman's heat-shield, watching him shiver and tremble on the floor reminding Batman of what the caped one has done to Freeze (in this version, Batman's responsible for Freeze's condition, by accident). This is the one glimmer of the future version of Freeze that fans are now familiar with.
Malachi Throne as False Face. This guy seemed genuinely threatening, perhaps due to the fact that you NEVER saw what his face really looked like. His general aura was just mysterious and dark, in contrast to the rest of the series.
I also liked Anne Baxter as Zelda the Great, since she was the only genuinely decent person in the Rogues' Gallery who really had no desire to be a thief. That and I think back then that Anne Baxter was hot.
And on that note, at the risk of veering into camp territory here, I liked Carolyn Jones as the Queen of Diamonds. I have to admit, seeing her do the Dance of the Seven Veils is something of a turn-on (more so when you recall what a great hourglass figure she had - I just watched "House Of Wax" for the first time in a while a few days ago and she was a bit of cutie in her day

).