Classic sitcoms/series often featured special episodes with songs or musical numbers - many unforgettable, both good and bad.

A few of my faves:

"You Need Us'' - The Honeybees (Gilligan's Island)
"I'll Blow You a Kiss in the Wind'' - Serena/Boyce & Hart (Bewitched)
"Eep Op Ork Ah-Ah" - Jet Screamer (The Jetsons)
"I Ain't Gonna Be Your Fool" - Ann Margrock (The Flintstones)
"California Nights" - Pussycat (Batman)
"Johnny Angel" - Mary Stone (Donna Reed)
"Time to Change" - The Brady Kids

What are some of your favorites - or least?

Jeremy Clyde of Chad and Jeremy plays Greensleeves on an S8 My Three Sons episode:

https://www.youtube.com/watch?v=n_PoZZZq5Wc

Tina Cole sings part of Downtown on an S8 episode of My Three Sons:

https://www.youtube.com/watch?v=ZcEM_xdHe7c

On One day at a time , 3 episodes doing a floor show at a retirement home featuring Julie and Max singing "Love and marriage" and Barbara/Cuddles Cooper singing "Spend a little time with me"

Two episodes of The Joey Bishop Show have good musical moments:

The episode with Jack Jones where Jack sings "Wives And Lovers".
The episode with Andy Williams where Andy sings "More".

Gomer's "Impossible Dream" and Gilligan's "Hamlet ala Carmen" were good for me too.

And thanks for the link to Katie's "Downtown" - I'd forgotten it!

Some others I like:

"Pearl, Pearl, Pearl" - Flatt & Scruggs (The Beverly Hillbillies)
"Girl" - Davy Jones (The Brady Bunch)
"I Love Lucy" - Ricky Ricardo
"I Wanna Be Loved By You" - Ginger Grant (Gilligan's Island)

Katie sings another song, one that Don Grady wrote, on a season 12 episode. MeTV cuts it out. Only my opinion, but it's not very good.

Yes, Nabors really nails the Imposs. Dream. Probably his best performance in the series. Later he teams up with Rob Reiner and the hippies to do Blowin' in the Wind, which was also pretty good.

Night Court

Dan playing Mairzy Doats on the accordion

Christine playing Two-Part Invention No. 3 in D Major