Hrm. Hard to know how to do this question, if you include TOS. The budget was simply small, and production standards of the time were what was expected. The various, “Western gear” was as good as the costumes for Bonanza or the Rifleman or The Lone Ranger.
How about this: Elaan of Troyius – France Nuyen wears a simple, revealing costume, yet she’s “dressed up” in her stilted accent. She brought her costume with her. And they do it again, for Ilya from the movie, which was based on a character that was supposed to be an early reboot of the series, that then became the character of Deanna Troi for next-gen. Same deal, we dress you reveling, you talk stilted, instant alien.
Truly, I had no idea Marina Sirtis was a British actress of some renown – I really thought she was a foreign actress who didn’t speak American English well. She could’ve been France Nyguen’s daughter for all I knew.
OK, in before everyone else, not that I was complaining about the costumes.
How about this, the Orion slave girl makeup for Yvonne Craig. Again, not complaining about the costume – but as she dances, you see some unpainted skin. It kinda took me out of the suspension of disbelief, even as a kid. Paint her everywhere (oh, grow up) or fix the costume to cover where you can’t paint (sigh, grow up) or don’t have her move so much (cut.it.out.) Its just a lack of dedication to the Star Trek alien meme they were trying to create.