Assuming you are a of a certain age group…even if you forgot every other episode of a show there is always a certain episode (or group of episodes) that nearly everyone remembers
Everyone remembers the Big Foot episodes of the Six Million Dollar Man.
Everyone remembers the Space Vampire episode of Buck Rogers.
Other examples?
WKRP in Cincinnati: the Thanksgiving turkey drop.
Newhart: the finale, specifically the final 2 minutes, when Bob’s character Dick wakes up from his dream, to find himself in bed with Suzanne Pleshette.
WKRP In Cincinnati - “Turkeys Away”
Yes, the series had other memorable moments…but let’s face it, “Turkeys” is and will always be easily THE episode everyone remembers.
Edited to add: LOL…and just as I feared, I was ninja-ed.
Mary Tyler Moore Show: “Chuckles Bites the Dust.”
Four words: Chuckles Bites the Dust.
All in the Family Rashomon episode.
Jonny Quest’s ‘Electrical Monster’ ep.
*Only Fools and Horses * chandelier drop.
The MAS*H finale.
How about the Family Ties episode where Alex P. Keaton is visited by the Christmas ghosts like in A Christmas Carol. I didn’t watch that show, and only have a passing familiarity with the characters and situations, but I’ve inexplicably seen that one episode - and only that one - three times!
“He crushed the wheelbarrow!”
“Our only means of transportation …”
Gilligan’s Island: Toss up: The one with the astronauts, and the one with the cosmonauts.
Star Trek: “The Trouble with Tribbles.”
Bewitched: Toss-up: The one where Darrin can only speak Italian, and the one where Darrin can only speak the truth.
Wild, Wild West: The one where Harvey Korman is an evil Prussian diplomat.
The Carol Burnett Show: the spoof of “Gone With The Wind,” in which Carol / Scarlett wears a dress made from curtains, complete with the curtain rod.
The Carol Burnett Show: Went With the Wind! I was in middle school when this first aired and I remember laughing so hard I couldn’t breath when Scarlet came down the stairs in that dress and then seeing my whole family around me wiping tears of laughter from their faces.
ETA: Beat me by one minute!
The Sopranos: The one where Paulie and Christopher spend the night in the woods. (Whatever happened to the Russian? 
)
Cheers!: The one where Coach’s daughter comes to visit. My all-time favorite episode!
Frasier: The one where Daphne hooks up with Niles’s doppelganger.
Seinfeld: The Contest
The other Cheers episode I’d suggest would be “Thanksgiving Orphans,” the one in which they all gather at Carla’s house for Thanksgiving dinner, which devolves into a food fight.
Now, let’s see if this post ninjas someone for the third time in this thread. 
Another Gilligan’s Island episode is the one with the Jack-and-the-Beanstalk dream sequence … where Mary Ann wears that French maid’s outfit. :o
No way. I know it was watched by 2 billion people at the time but it’s largely forgettable. I would say the one episode people remember is when Henry died.
Magnum P.I. “Did you see the sunrise?”
I Love Lucy - The Candy Factory episode followed by Lucy at the Italian winery.
The Bionic Woman - The one with the fembots
The Twilight Zone - Eye of the Beholder
Happy Days-The one where Fonzie jumps the shark 