Moving further back in time now to that primeval wilderness inhabited by the NES, Atari 2600, Colecovision, Master System, and others including various home computers and the arcades, what do you think was the very best video game of 1980-1989, and why?

Id give it to Mario Bros 3.

Acouple of my personal favorites below,

Carrier Command - Was one of the greatest games ever made and the most ambitions at the time. Simulation on a Amiga/Atari.

Speed Ball - Was just so much fun, back in those days where there were no Fifas and Loot Boxes and Gambling, just hardcore sport action with a good team management system in its days.

Speed Ball 2: Brutal Deluxe - Did release in 1990 however I still like to consider it the last of the 80s games. It was developed on 80s hardware.

Maniac Mansion.

Funny story and dialogues, great humor, unforgettable characters, great puzzles, co-op puzzles by switching between characters, good replayablilty by choosing different characters. Good graphics and awesome music for its time... also microwaving a hamster.

Ah. Barely in time to choose Super Mario Bros 3.

Pac-Man.

Most influential game of its the decade. It started Mascots in games. It created Pac-Man fever. First video game to have a kids cartoon I believe.

Not my personal fave but the game was so influencial. Only thing that matches it, Super Mario Bros on NES not the arcade game.

The Legend of Zelda. At the time there was nothing else like it. I'd dabbled in computer RPGs before, but Zelda was the first game I ever got into on a long-term basis. The Zelda series is my favorite video game franchise to this day.

Mega Man 2. It just perfected the formula. The gameplay, graphics, music and everything else about it is top notch.

Mario Bros 3. No need to justify, elaborate or explain that answer.

Impossible Mission

Mixture of platforming, adventure and puzzle genres.
Random room layouts made every replay different.
"Another visitor. Stay a while... stay forever!"

Aaaahhhhhhhh~

I can't understand why nobody mentioned Indiana Jones, this is a real classic:

Arkanoid, Battle Chess or Populous are also classics. And nethack was from the 80s, the game that was the first popular roguelike (after rogue itself).