The thread title is so beautifully simple I can’t bring myself to complicate it as I suuallywould. Instead I’ll just start off by mentioning Dr. Elizabeth Rodgers from Law & Order (and Law & Order: Criminal Intent), portrayed by Leslie Hendrix. She’s by far the most believable medical examiner on any of the cop shows (sorry, Ducky!) and has a believable, human hotness about her that the parade of model/ADAs on that show do not.
Anybody else?
The Simpsons is the gold standard for this, IMO–too many to pick one. So how about Sidney Freedman from MASH*? Only appeared in 12 shows, but even a casual MAS*H fan knows Sidney.
Creed from The Office. Just a few moments of total insanity.
Bob and June Wheeler from Night Court.
Kelly, Woody’s fiancee from Cheers.
Eko from Lost.
Doctor Miquelito Loveless from The Wild Wild West. The unforgettable Michael Dunn.
I love Loveless too, but he’s not a minor character. He’s the major recurring antagonist, and the focus of every episode in which he appears.
2 1/2 Men had psychiatrist Dr. Linda Freeman (and boy, they missed a great opportunity to have her father Sydney appear), Criminal Minds had Spencer Reid’s crazy, institutionalized mother Diana. Now she’s Sue Sylvester on Glee. Here’s to the amazingly talented Jane Lynch!
Get your cotton-pickin’ hands off! She’s MINE, dammit!
I’m also fond of ****Janet Fraiser on Stargate SG-1. Teryl Rotheray has the most beautiful eyes.
She probably doesn’t qualify as “minor” but, back in the day, I had a major thing for Zoey Woodbine (Alicia Witt) on Cybill. Marvelously snarky, and a genius-level intellect. Extraordinarily attractive on those grounds alone, even without considering her nuclear-level hotness.
Ernest T. Bass from Andy Griffith. Hard to believe he only made five appearances over the entire run of the show!
I’m going to take that as a hookerbot request and have instructed Accounting to bill you accordingly.
Kathleen Turner as Chandler’s often mentioned, but never seen till Chandler & Monica’s wedding, transvesite father James Bing. Her two appearances were unforgetable.
Chandler: And there’s Daddy!
Monica, totally dropped jaw: That cannot be your father.
Chandler: Believe me, I’ve been saying that for years.
Daddy: It’s bad luck to see the bride in her wedding dress before the ceremony.
Mommy: I sae the groom in my wedding dress.
Daddy: Yes, but it was after the ceremony, so it wasn’t bad luck.
Mommy: Well, it wasn’t good luck.
Every time Richardson was on screen in Deadwood I cracked up. “Is it possible you were mother-born and not egg-laid as I have always imagined?”
I always thought MASH’s Colonel Flag was a classic—The first couple of times he was featured, it was strictly all laughs, but later he was involved in at least one dramatic episode, where he executed a Korean woman guerrilla fighter, with Hawkeye helpless to stop him…
Peter Jurasik as Sid the Snitch on Hill Street Blues. Who knew he would go on to be Emperor of the Centauri?
Brent Spiner as Bob Wheeler on Night Court. What was supposed to be a once shot character was brought back several times.
David Morse as Detective Michael Tritter on House.
The incomparable Hank Azaria as Murray the dog walker on Mad About You and Phoebe’s beau David on Friends. Both roles end him Emmy Nominations.
Uncle Arthur from “Bewitched.”
Dualla from “BGS” - mainly for her exit from the series.
Comic Book Guy from “the Simpsons.”
Sally Sparrow from “Doctor Who.”
How about the recurring Janice? “OH…MY…GAAAWWWD!” 
John Astin as Buddy, Harry’s step-father on Night Court. He was in 10 episodes, but he was my favorite character. Even more than any of the regulars.
Loved his catch phrase…"[Some awful thing happened], but I’m feeling much better now."
Damn! Beat me to it.
Also from Doctor Who, Reinette from “The Girl In The Fireplace”.
Brittany from Glee. Show’s worth watching just for her absurdities. Heather Morris is pretty, but it’s her deadpan lines and her amazing dance moves that sell Brittany.
Oh, yeah - we still say that.