Probably Rimworld. With the storytelling aspect, each playthrough feels like a different experience.

I'd go with "Hide the Sausage".

Minecraft. Mods are like magic, and can totally change the game. I've lost countless hours just poking around and building fun things, even on vanilla.

Chess

I have about ~9,000 hours logged in Skyrim over the last twelve years thanks to mods, and I'm sure I'm not done. So I'll stick with that.

For me, it would definitely be Red Dead Redemption 2. I've played the main story several times now and will probably soon start over again.

The Mass Effect trilogy (I consider it one game) - no matter how many times I play it, it still feels fresh.

Skyrim

In fact, this is the only game that I have played for the last 6 months lol

Rimworld with mods. The shear amount of wild and bizarre experiences that I have in that game could easily keep me entertained for a lifetime. I doubt I’d get bored with it. Other than Stardew Valley and Zelda BOTW, it’s the only game I keep coming back to and unlike those two games, I don’t get bored of it. I simply have to tear myself away from it so I can function like a normal human being again.

Probably RimWorld. Somehow I've managed to accumulate nearly 4,000 hours. I'm pretty sure I could play for another 4,000 easily and then for the rest of my life. I love the stories that come out of each colony and how attached I get to pawns. I'll never forget Dinkleberg, the troublesome cat that has nearly wiped my colony twice by leaving doors open for invaders then when out by snorting too much cocaine

Rocket league. I know some people consider it the same thing over and over, but to me it plays like a sport. Every match is different, I've been playing almost daily since the pandemic, which is when I got into it. And apparently I'm in the top 5% of players. Which just shows how much the game clicked with me

Let's see how Starfield turns out to be. If it is any good, it will entertain me for thousands of hours and if it has good mod support, why not commit for it for the rest of my days? Still a lot of ifs tho

Guild Wars 2

I would love to say Crusader Kings, but it's probably chess.

Age of Wonders 4, the sheer replayability of the game is bonkers. And they will only release more content as time goes on, both through DLCs, free content updates, and workshop mods.

Cribbage, mainly because I like calling out 'one for his knob '.

Rocket League

And I'm gonna hate the rest of my life

Minecraft, while nothing particularly special to me, is most likely what I would choose. The potential for making your own fun in that game is ridiculous even if its been the victim of bad new mechanics bloating vanilla for the past decade. Even the longest of my favorite games cap out around 200-300 hours with all their content squeezed out of them, and with the amount of gaming I do on the regular thats gonna last me a few months at best.

Dark Souls remastered

Honestly, yeah. If I'm allowed Mods, then the roof is just the level of my creativity. If I'm only allowed one game, it's a damned good one.

Minecraft with mods definetly - really adds a lot of variety, and even though I take long breaks i've been playing it a ridiculous amount of time since early beta maybe 11 or so years ago.

Can I add Mods? Then Skyrim VR.

If I can't, I'd choose a Game as a Service type of deal, if we could also make the assumption that the game would also be actively developed for the rest of my life.
Destiny 2 comes to mind, because I really love the lore and the gameplay.

If we can't add mods or the game won't be updated for the rest of my life, I'd still choose Destiny 2. After it gets cancelled, I'd just go outside and touch grass then, I guess. 😅

Age of Empires 2. I am playing it for 24 years already, so why not 50 more?

Probably Diablo 2 LoD. Best ARPG ever made.

Nethack. But definitely not in vr.

Elite Dangerous, assuming the servers would stay up I have an entire galaxy to explore

For me it's Assetto Corsa (modded, in VR). Most-immersive, addictive, exciting gaming experience possible, in my opinion. I did thousands of miles on the Tokyo interstate at night alone, just trying out different cars. Sometimes raining. Sometimes not. Listen to music while you drive/race. Really amazing.

Skyrim with mods, honestly. I feel like there's a lot of potential there

The Binding of Isaac: Repentance.

Before I had kids: PUBG. I went real hard on that game for 18 months. I played 10+ hrs a week and never lost interest.

After kids (PUBG is stressful, I don't need more of that) it'd be Rocket League. I've been playing it consistently since Day 1 and whilst I don't see myself improving much anymore, I still get a kick (ahem) out of scoring a nice goal, or a laugh at totally missing an empty net.

Probably path of exile. I've already put so much time into that game and still have no idea what the hell I'm doing, I think I could easily put a lifetime in without trying everything.

With friends, it'd probably be World of Warcraft. I already have 20k+ hours in the game, and would likely play it even now, if Blizzard wasn't just complete garbage right now.

Solo, a Diablo game I think, although I don't know which. I haven't played D4 (and won't for a while, same reason as I mentioned above), I love the fast and fluid combat in D3, but D2 might have more variety.

Elden Ring is a contender for sure

TheoTown, it's such an underrated mobile game and i would love to just play it for the rest of my life

Dota. It's a game with infinite depth and variety. A game where the first 1000hours are the tutorial. A game where there are A LOT of players with over 10k hours in it. Nothing compares with its highs and lows.

Binding of Issac, for me one of the few games I could probably continue playing for a decade and still enjoy.

Slay the Spire is a strong contender, but overall the Isaac devs seem more like they would be swayed to create even more content for me.

Oh wait I forgot about MMOs, maybe FFXIV is a better choice in that case, I just forgot about it cause I am taking a long break until the next expansion.

I would probably say morrowind with all the community mods and modern engine.

Considering how many hours I’ve lost to FFXIV, I think that’s gonna be my choice!

Honestly, I already rotate through maybe 4-5 games. Skyrim or Minecraft could probably hold me over.

Street Fighter 6. Hell, i know some who will only play that for a few years. FGC be damned.

Edit: Whoops, misread and thought it was for a year. I change my answer to Unity, lol.

At this point, I think I might have to go for monster hunter. Or Dragon's Dogma. Just some kind of super deep action RPG would do it.

I've stopped playing it for years but if there is only 1 game I can play, it will be Dota2. In fact I stopped playing it because it took all of my time from other games

Could probably be happy with Hades

Although if Halo MCC count as one game...

Single player - F-Zero GX. The skill cap in this game is astronomical - mastering it akin to mastering a musical instrument. There's nothing else quite like it.

Multiplayer - Smash Ultimate. The amount of unique character matchups and interactions is so massive that there's always something new to see.

Path of Exile.

If I was never introduced to today's graphics, easily Morrowind. I would still pick Morrowind over Skyrim.

Toss up between Diablo 2 lod and path of exile. Just an endless gameplay loop thats extremely satisfying to me

Assassin's Creed Valhalla. That game just never gets old for me. As an archaeologist I think this is as close to being in that time (albeit with it's anachronisms) as I will get.

Is multiplayer allowed? If so, probably Dota 2. For single player, Factorio.

Pull up a chair and let me tell you about a little game called Nethack.

Stardew or Skyrim since I can mod it for more content :D

FFXIV as a large majority of my friends play that game

League of Legends probably if we talk about video games.

Doom/Doom II. Already been playing it for 3/4 of my life, and the modding community still keeps it fresh so even if the core game gets old, there's still things to do with it.