I might be forgetting something but the main ones that come to mind are:

Secret of Monkey Island (1990)
Legend of Kyrandia: Malcolms Revenge (1994)
Command and Conquer (1995)

If limiting to games I played from floppy disks "back in the day", and games I owned:

Quest for Glory I (1989), I guess. I probably have older examples but I can't remember off the top of my head.

I suppose King's Quest III (1986) and Space Quest II (1987) may count, but only if you allow walk-through help.

What the hell, past me -- this wasn't even true eight years ago when I posted this. Without even considering any qualified cases like those below, I beat WarCraft: Orcs & Humans (DOS - PC, 1994) around the late '90s, and that's most of three years older than GoldenEye.

- If you count games only beat using a walkthrough, the Windows 3.1 release of Myst (1993 or '94, depending on whether you count go by the release date of the very first version for Macintosh, or count that first Windows port as its own thing)
- And if you count games that I and/or or my family used to own when I beat them, but no longer own, then The Simpsons: Bart vs the Space Mutants (NES, 1991). Also, this might be not just the only NES game I ever really completed, but the only such game for any of the consoles I had some access to from my parents, my friends, or via rentals before I started buying my own consoles.

If the oldest game someone's beaten is from the 2000s, then that person couldn't have beaten Doom because Doom came out in the 90s.

Anyway, ignore me...sometimes I get a little too obsessive and weird about Doom. In my mind, it feels like a game that's essential for everyone to play. But, also, yeah, I know that's not really true - everyone should just play whatever they feel like :)

Mission Kellogg’s

The original Dizzy (1987) on the ZX Spectrum was the oldest game I owned and had completed. Don't own either the game or the Speccy anymore, as the Amiga took pride of place a year or so later, with the SAM Coupe fitting in between somewhere during that time...

For real? Did you use the magic ring trick by chance?

I tried the game for several times, I think it's the most random hardcore RPG game I've ever tried. I don't know what was hardest: to not die out of starvation or to please Lord British.

Super Mario Bros. 1985
The Legend of Zelda 1986
Phantasy Star 1987
Final Fantasy 1987

Question by "still own", does stuff stored in mom's basement count? Also are we talking consoles or computers?

A lot of the console games were not beatable, as they would just loop the levels endlessly.
Still, on the Intelivision, there was Sub Hunt, Sea Battle, Star Strike, Utopia, Backgammon, Poker & Blackjack, and Space Battle. I'm not sure about including Auto Racing considering you got unlimited cars to complete the course and just competed for time.

On the computer, are we talking about "oldest" by release date or purchase date? If purchase date, it would probably be Skyfox, as there were 3 games bundled with the computer when my parents purchased it. Skyfox, Sea Dragon (which I never beat even on "easy" mode), and Dig dug. Dig Dug could not be beat as the levels would just loop endlessly.

You can play COTW on Windows pretty easily. Just extract the package below, and click on the .bat file for COTW 1 or 2.

Castle of the Winds - WineDVM 2022

DONKEY KONG II on the Nintendo Game & Watch in 1983.

https://www.youtube.com/watch?v=rEno8A2WM_w

The first video game I played was PONG on a PONG home console in 1980.

It's not a ring; it's an amulet.

I was playing the DOS remake, which has a save feature. In a game where the amulet has a chance of boosting your stats, the ability to save and load is easily exploitable.

With that said, the save feature isn't necessary to do this. The game's RNG is seeded to a value based on user input (in fact, when the game asks you for your lucky number, that number is used as the seed), so it's possibly to manipulate the RNG so that you get lucky when you first use the amulet, by just repeating the same inputs every time and maybe wasting another turn if you don't get the effect on the first attempt. In fact, if you want to learn how RNG manipulation works, Akalabeth would be a good game to start.

I would argue that you could count such a game as beaten if one of these conditions are met:
* You've reached a point where the level counter overflows, causing the levels to repeat from the beginning.
* You reach a point where the levels stop changing, being identical on each room.
* You have reached an impossible level, also known as a "kill screen". For example, in Donkey Kong, the time allowed for each level keeps increasing until it overflows, at which point the time limit is so low that it is impossible to progress any further; at this point I consider Donkey Kong to have been fully completed.

I apologize, it was so long ago that I confused the ring with the amulet. Yes, I've heard about the seed number, but I never understood how implement it properly. I get it that without the amulet, lucky seed number and save & load exploitation it's almost impossible to beat. Akalabeth would be a good start since I longed to check out Ultima series from the very beginning.

Yes, the amulet. I secretly hoped that someone beat the game *properly* and would tell us how it was.