"Mudd's Women" - TOS 1X3 - Trekkie Feminist
trekkiefeminist.com
If "Angel One" is “the sexist episode” of The Next Generation, then “Mudd’s Women” is one of the strongest contenders for that title in the original series.

note that the link to the Angel One review is now age-gated on tumblr; it can be read here on archive.org or on the blog’s own domain here.