No one else has posted this yet, but it's my inevitable choice:
Sabrina Scharf as Miramanee
Not entirely her fault, perhaps, given lines such as "I have nothing to fear; you are here," "...the strange lodge which moves through the sky," etc. Despite her sporting what I believe to be the largest chest measurement of any (female) guest star, she simply was not equipped to breathe life into lines like that. (Who would have been, I wonder?)
Yup, she was bad. Forgot about her. My kids like almost every episode uncritically. Not that one, though.