For me it's Chrono Trigger. I always want to play it. I want to show it to my children. I hope it will be regarded as a masterpiece for generations to come.
DooM (1993), but mostly because of the community. Constant new levels, mods, engine updates, total conversions. Even commercial games released on the engine. It just never stops.
First off you're gonna want to grab yourself a source port like GZDoom so you can run it on a modern PC with all the extra bells and whistles. Go grab the original DooM and DooM 2 off Steam for $10 each if you somehow don't already have them. A lot of stuff still needs access to the original .wad files.
While I do love the things people have done with GZDoom, I always find myself coming back to a more classic doom experience. Something like dsda-doom that tries to stick to rules of the originals is more my jam, and it has some nice quality of life features that don't alter the core gameplay. There is just something about the basic and simple mechanics of base Doom II that is timeless.
If you are looking for quality WAD recommendations, there is a continually updated Doomworld Community Top WADs of All Time or you can check out the annual Cacowards that attempt to acknowledge the best new doom content created every year. Many of these wads dramatically outdo the originals in terms of map design. Eviternity—a favorite of mine—which comes in at #6 on that list actually had it's creator scouted to do the maps for the boomer shooter Prodeus .
Also, even though I prefer the classic experience, my favorite weapon mod has to be Final Doomer. Brutal Doom always gets all the hype, but this one deserves a lot of love for the fact that it tried to add weapons to fit specific wads. It gave the wads it was made for so much more flavor, and, while definitely more powerful than vanilla weapons, it wasn't to an absurd level like brutal doom.
A good level in that game goes harder than any current AAA fps I can think of. Probably the remakes get pretty close, and they blew everyone away because they were a massive improvement to the fps formula, but idk if I'd ever call the remakes "timeless".