Day 1-A show that never should have been canceled.
I have a couple of answers for this, all dating from my formative years in the mid 70's.
One is City of Angeles, starring Wayne Rogers, after he left M*A*S*H. It lasted one 1/2 season in 1976 and I remember being absolutely in love with it. It may be the first series that I was in love with and aware of being canceled. Sort of a "lighter" Chinatown with Wayne Rogers as a 30-ish tough guy detective. Massive crush, and it just makes me sad that NO ONE remembers this and I can't even find a You-tube clip to show you.
Another is a very short lived Norman Lear sit-com called HotL Baltimore. Another crush situation. Please don't laugh....Richard Masur. I'd thought he was actually very hot (to my pre-teen self) on One Day At A Time. His character was generally very wimpy compared to Bonnie Franklin's powerhouse, but there's this one episode where she catches him bed with another woman, where he was just...very sexy.
This show was super-controversial at the time because it had prostitutes and a gay couple. (Of course the gay couple was in the "Staircase" mode, but still...) I remember thinking it was very funny and being bummed when it got canceled. Another one that only last 13 episodes.
But the winner and champion...and vanillafluffy knows it's coming... Baa Baa Black Sheep which became Black Sheep Squadron, starring the incredibly hot Robert Conrad, back in his "I dare you to knock it off" battery commercial days. Obviously the show couldn't have gone on indefinitely, or it would have gone into M*A*S*H territory, exceeding the time the war actually lasted, but it could have gone on a few more seasons. Such an awesome cast, full of guys who were on their way to bigger things: John Laroquette, Larry Manetti, James Whitmore Jr, plus great supporting actors like Simon Oakland and Dana Elcar. Conrad did another series afterwards "A Man Called Sloane" where he tried the debonair, spy thing and it went no where, although I think I watched every episode of that too.
I still think of the show a lot because James Whitmore Jr. has done so much directing that his name shows up at least once a month directing an episode of something I watch. (Bones, NCIS etc.)
Here's a clip for you: