OP, I don't think you understand how big of an oversight forgetting Burgess Meredith is. It's literally the answer to your question.
Burgess was a very big actor at the time and his Penguin was so popular that it carried the character into the limelight.
Think about all the other Batman 66 villians, there's tons of them and most are obscure.
The thing is, I kind of disagree with this.
First of all, you have to remember that the character was created in
1941. So first, you have to ask yourself what exactly it was that made the character so memorable that he was selected as one of the main bad guys for the Adam West show twenty years later. What was it about The Penguin that the producers liked? Why was he selected over the likes of say....Two-Face or Hugo Strange? What was it the producers saw in the character?
But even dismissing that, there's still the fact that the 1960s Batman show wrapped up around five
decades ago. That's what, three generations back? And yet the character is still just as prominent and iconic today as he was back then....being utilised just as regularly in the comics and in the vast majority of all of Batman's adventures in other media, increasingly now by people who never got the chance to grow up with the show.
Primarily, I wanted this thread to explore what makes this oddity of a character interesting, what makes him stand out as a villain.... and yes, also what's allowed him to continue to endure throughout the passing of the decades to a point where he's still considered one of Batman's most iconic villains. I don't think just saying "Burgess Meredith!" fully answers that question, although I recognise the actor has had a lot to do with ol' Pengy's staying power.