I could quibble, I suppose, and claim that she was only in the episode as a token villain, not a fully realized character, but that would be small-minded of me.
But the claim isn’t that she was the first female captain in Starfleet, just the first to be shown on the show.
Garret was (possibly*) the earliest shown, within the series timeline, but it took them two years longer to show her.
I wouldn’t place a bet on either side of a discussion of if we’ve seen any female officers from before her in flashbacks (or spin-off novels/comics). I can’t think of any, but that’s a lot of material one could have appeared in, and I may be forgetting.
I always loved the single episode that Wolfgang Puck appeared in Frasier - the one where Niles sets up an elaborate engagement scenario with Daphne which backfires due to her getting sick.
Same series - but Michael Keaton’s one-off character of Lilith’s brother was very memorable as well.
Heck, Frasier had a ton of great one-episode characters.
As has been pointed out, Shelby’s not even the first Starfleet command-level female officer. In addition to Joanne Linville from the already-mentioned TOS episode The Enterprise Incident (the first female commanding a starship, albeit a Romulan one), there was Tryla Scott (a full-rank Captain) in the earlier TNG episode Conspiracy but even before that, Uhura took command of the TOS Enterprise in the animated episode The Lorelei Signal.
On a semi-related note, Patrick Stewart’s guest shot as Number One in the “Stonecutters” episode of The Simpsons always kills me.
Patrick Stewart played Number One?! I never knew that. Wonderful on several levels.
Lord Flashheart on Blackadder–does he count? I know he came back for the fourth one, and the movie, but he was only in one episode of the second series and he was nothing if not memorable.
The Big Bang Theory Lewis BlackProfessor Crawley He was hilarious as the laid off Entomologist
The Big Bang Theory Riki LindhomeRamona Nowitzki She was great as a grad student with an obsession with Sheldon
The Big Bang Theory Mayim BialikAmy Farrah Fowler Dating service finds female Sheldon. I figure I better mention her now, since she is supposed to be back next month.
They also had some other great guests in Laurie Metcalf, Christine Baranski and Wil Wheaton, but they have already been back for encores.
Michael Keaton’s one-off shot on Frasier was indeed excellent, and Patrick Stewart also starred in a great episode where he was romantically pursuing Frasier…
Tony Shalhoub also did a guest shot on Frasier that was perfect.
Peter Boyle’s appearance as Clyde Bruckman on the X-Files episode, “Clyde Bruckman’s Final Repose”. He brought a lot of pathos to his role as a reluctant psychic, and the episode is among the X-Files’ best.
Wow, I’m not even much of a Whovian but the first thing that came to my mind when I saw the Subject line was Sally Sparrow on the Doctor Who episode Blink, as played by Carey Mulligan. Here’s a trailer for the episode.
It’s considered by many one of the greatest Doctor Who episodes ever. So where are all the DW fans?