The oldest I can go reliably is the late 80’s and early 90’s.

DuckTales NES, Sonic the Hedgehog Genesis, Puzzle Bobble.

Once we start getting into the mid 90*s games start to get good.

Chrono Trigger, Doom, Rayman, DK Country

Garrys Mod. You'd actually be impressed how many people still play that game for it approaching its 20 year anniversary.

I've been playing jet moto with my kid. Just like I did with my dad, back in 1996.

I still play a phone sudoku game from 2009. I would still play StarCraft and Warcraft 2 if I had hardware capable.

Every so many years I fire up a King's Quest or Space Quest.

Mario 3, though these days I get tired of it after a World or two and haven't completed it in a while.

Also Xenogears from the PS1 is still one of my favorite games and I replay it every few years.

Anarchy Online (2001) is a game I play frequently.

Mario Kart: Double Dash!!

About once a year or so I go replay Morrowind, the chaddest of the Elder Scrolls series.

This past year I replayed Morrowind with Tamriel Rebuilt tacked onto it, which has been extremely cool and fresh. TR really feels like official content a lot of the time, it's quite good and the love that's been put into it is very apparent.

I play lots of games from my childhood. Even though it was before my time, Super Mario Bros on the NES is probably the oldest game I regularly go back to. But I love retro games, so I'll play even older games on occasion.

Through the Ages, the app edition of the board game.

I play that at least twice on a long flight.

I'm on a retro game kick lately. I am currently playing Pokemon Leaf Green and Chrono Trigger (for a fiesta event) and will likely move on to Castlevania: Symphony of the Night afterward.

The barcade near me has a Sinistar (1982) cabinet and I play it every time I go. I’m not even that good at it, but I love it so much.

Also I love those Lucasarts point and click adventure games. Grew up playing Monkey Island and play it again every once in a while.

Freeciv

I replayed Vampire: The Masquerade — Bloodlines (2004) recently

Gothic 1 and 2 (from 2001 and 2002 respectively) are still some of the best games I ever played. There is also a very active modding community, and some mods are even much better than the original game.

Ive played through Kingdom Hearts every couple of years since it came out! I was in 1st or 2nd grade when it came out and it was one of the first "big" games I ever beat.

Started playing Spelunky HD again the other day, the sequel is better but the original is still fun to revisit.

Morrowind and Fallout: New Vegas sometimes. I've tried playing the original two Fallout games but I keep bouncing off the first hour or two.

Some Guilty Gear XX AC+R with a friend -- we would love to play some old Tekken games too but we're both on PC so Tekken 7 is the oldest available.

Every once in a while I'll play some Sacrifice, such an amazing game that's dying for a remaster.

Regularly Cube2:Sauerbraten, initially from 2004.

Occasionally, Dragster on Atari,1980. Some of the best reaction gameplay available, from the golden years.

Mario 64 on original hardware. Never did finish it as a kid so no time better than the present

I still play NES games often. These were the games of my childhood. Balloon Fight and the original arcade Mario Bros. are always fun for shorter sessions. And I still enjoy playing through Super Mario Bros. on occasion. I very rarely have ventured into games older than the NES.

Jak X.

Does OpenTTD count?

On a regular basis, probably classic DooM / DooM 2.

Last three were Star Ocean (the first one), Radiata Stories, and Skies of Arcadia.

Currently playing Alundra because a friend grew up with it and wanted me to play it. Honestly it's not great but RetroAchevements made the bosses extraordinarily painful to master, which is uh ... fun. Yes. Fun, let's go with that.

Oh and ps these commenters all have fantastic tastes.

I had to check my library to make sure which one is the oldest, but it's fallout 1 :3