What game is worth owning forever so you can spend hundreds of hours without getting bored?

Are you asking for personal advice, or, are you simply attempting to gather opinions from community members?

Either way, my definitive answer would be: Sid Meier's Alpha Centauri

While experiencing this fantastic game, with multiple layers of depth (that has yet to be matched, let alone surpassed), I have cherished each and every moment. The project videos evoke a torrent of emotions, even after having viewed them hundreds of times.

Of course, there are other games that provide long-term value (rather than the typical one-time play-through), but, in my opinion, nothing approaches the greatness of Alpha Centauri.

Anything with a random "seed" aspect to it (to generate the traversable world at the beginning of each new playthrough, for instance) should fit the bill.

Battle Brothers being a prime example.

Or anything where you can choose from a variety of different backgrounds/origins/circumstances you start out as/from, like in Kenshi or, coincidentally Battle Brothers as well.

Honestly, is that the best you can do?

As a strategy enjoyer, i would eagerly point towards the nice collection of strategy titles present on GoG. But the thing is.... it seems there are also people who apparently can enjoy hundreds of hours in games such as Doom or Skyrim for example. Especially with the huge support from the modding scene those titles enjoy.

Still, a strategy title would be my personal pick.... Shadow Empire for the sake of this topic

Give it a go

I have to say my most played game ever: Civilization IV. Genius game design with multiple systems that interact with each other uncovering synergies, a technology tree with several paths to choose, many ways too victory... in infinite maps and situations. I "confess" I spent a decade playing almost exclusively this game —I didn't need another one.

And when you get tired of the vanilla game, you can explore the amazing mods with professional quality, such as the wonderful Fall from Heaven II. That's like getting new games for free.

Pathfinder Wrath of the Righteous is like 120 hours with probably 20 very unique playthroughs.
That's hard to beat for a single game.

Though the Legend of Heroes: Trails series is my personal favorite. It has 12+ games that tell a continuous story.

Any of the 4X games. If you like them, a 100 hours is basically nothing.

Then of course ARPGs. Be it Titan Quest, Grim Dawn, Diablo or any of the countless others out there, again, if you like the genre, 100 hours feels like nothing.

Some longer RPGs can easily take 100+ hours per playthrough. Like Bethesda Fallouts or Gothic 3.

And of course, games intended to be replayed over and over with random generation seeds every new "run".

Mount & Blade Warband, on itself it has a lot of content but it becomes ludicrous once you add in mods.

Maybe the Stalker games as well.

Stardew Valley

https://www.gog.com/en/game/doom_doom_ii

Very much depends upon your taste in games.

For example I personally wouldnt even play civilization once. The very game concept just bores me.

And there are of course other games which other people actually play for hundreds or even thousands of hours, like for example the The Elder Scrolls games, but I was never motivated by them this much. In case of Oblivion amusingly I had more fun watching the YouTube ManyATrueNerd playing the game than with any of my own games. He tried many things I never even considered doing; when I play TES I frankly mainly fight with the annoying rulesystem.

These are the games I replay, some of them since 20 years, and I could get all of them on GoG:

- Baldurs Gate 1+2: The number of characters you can make is of course very high, and you can also do all kinds of other choices. The main story though is not really that flexible, even if for long periods of time you can freely travel the world. Either way especially BG2 is probably my favorite game of all times. Theoretically you are allowed to play as evil character but its quite the PITA, because the mechanisms for that arent too great. Both games plus the addons easily surpasses hundreds of hours; I need like three weeks or so playing through the series (not during holidays).

- Vampire: The Masquerade: Bloodlines - a very atmospheric game. Often I think this is actually my favorite game ever. You can play as one of seven vampire clans. Theres first the "mainstream" kind of vampire, which are mostly human and play kind of alike. Thats Toreador (Artist), Brujah (Warrior), and Ventrue (Polititian). Then there are four clans which play increasingly different from the "mainstream experience": Tremere (Mage), Gangrel (Animalistic Warrior), Nosferatu (Extremely Ugly Stealthusers) and finally Malkavian (Madmen with the ability to see the future and the truth), who even get their very own dialogues. The game only lasts like 20 hours or some such.

Oh I forgot: one of the main reasons I enjoy replaying the game is because the unofficial patch is even today still worked on, so the fans are still making the game better and better.

- Baldurs Gate 3: THis is a different company and it released more than 20 years after the originals. But with D&D5 instead of D&D2 (aka AD&D) we have a three generations later rulesystem (D&D4 was a bust though). I have yet to manage to play the game fully though, I estimate that requires around a week of holiday time ?

All in all you have much more flexibility in BG3 to make characters and all classes are more interesting than in older versions. But the magic system isnt as complex anymore and you get far less spells per day.

One of the things they want to add is better evil play. They also want to support modding better. So the game is still not fully done. It will probably soon be finalized though.

- No Man's Sky: If you know what you're doing I would estimate you would nowadays need about 150 - 200 hours total, at least, to maximize everything to a point at which further progress is only possible in very small amounts. If you dont know what you're doing it can easily last twice as long or longer.

- Star Wars: Knights of the Old Republic: For some reason I like replaying this every now and then, because its so well done and has such a great story and characters. I cant name an objective reason why its replayable though. Game length is only like 20 hours (if you know what you're doing).

The Alpha and Omega of video games. If I had to chose one single video game to spend the rest of my life with, it would be this one, no question asked.

+++ Error. Null entry. Redo from start.+++

That depends on your personal demands. If yours are very very low, you can have lots of entertainment for years with a ball and a stick.
Talking from experience...