Wizardry 8 is good; there's so many builds and party setups to work with, plus the game isn't linear. (In fact, there's a pair of major quest lines where you only have to do one of them, and only part of the way, to complete the game.)

Neverwinter Nights EE. If you enjoy the general gameplay, you can keep playing it for all eternity, trying all the different classes and multiclass combinations, playing and replaying any of the long campaigns and shorter DLCs, alone or with others, playing any of the hundreds of different free downloadable community-made campaigns and adventures or on MMO-like community-created persistent worlds, or you get creative with the easy-to-use toolset yourself and create your own adventures, campaigns or online worlds. In any case, there's enough content to keep you busy for decades. IF you enjoy the game.

Personally, I can't seem to stop playing the Batman Arkham games — especially Arkham Knight. The combat and predator/stealth sections are already pretty amazing, but the Batmobile also adds variety with some driving and shooting. It's pretty hard to get bored with it. :-)

I once played Wizardry 8 and I got pretty quickly bored of having to fight random encounters all the time, whereever I went. Literally the main advantage of leveling up was that you learned more spells and other abilities to avoid combat. But you still had to fight a lot and you could get a party wipe even on higher levels.

Otherwise I remember:

- Put Valkyrs on both sides because they can attack with their lances even if the enemies attack from other sides.

- Put Bishops in the center because while they start weak they WILL learn all spells in the end.

- Play exclusive all women parties because the game has women only items, but no men only items.

Not entirely true (though the male only items are considerably rarer).

What you have are:
* Chieftain's Sceptre: Requires killing a friendly NPC to obtain. It's a mace that boosts communication. (Interestingly, Valkyries can't equip it, not that it matters unless you somehow have a male Valkyrie (for example, by hacking).)
* Cameo Locket: A nice item, but it's one of those items that has only a small chance of appearing and can only appear in non-respawning sources (like chests).

Also, there is one particular quest where, if you have no male regular party members but do have at least one male RPC, you won't be able to complete it without dismissing that male RPC at least temporarily. (In 1.0, I believe that, if you had an all-female party, you couldn't complete that quest. The problem is, however, that, without foreknowledge of the game, this quest is basically mandatory, so the game could become unwinnable, hence why they had to fix that.)

I spent 500+ hours in Slay the Spire hitting Ascension 20 for everybody.

In, "Order I thought of it"
Rimworld.
FTL
Monster Train (I consider this one better than Slay the Spire, but others prefer Slay)
Old World (Or Civ4; I've finally replaced that with Old World in my mind)
Stardew Valley
Fell Seal

I'd say, that depends entirely on the person playing said game.
Because everyone has their own games to replay and spend hundreds of hours in.

I second Old World. It is designed by Soren Johnson, Civ 4's lead designer, and it has an equal excellency and addictiveness.

Witcher games, ThroneBreaker, Dragon Age: Origins, Baldur's Gate 3(I haven't played it yet, but I've heard a lot of positive things about it), KOTOR, KOTOR 2, Batman Arkham games, Fallout games,

Just to mention another obvious candidate, thats Dwarf Fortress.

To me it never quite hit home but I know at least one guy who enjoyed it greatly. I dont know if he actually put hundreds of hours into the game though.

Its of course freeware though.

About RimWorld, I have played with the thought of getting that for a long time, but:

- 2D is just a far too hefty limitation for a base building game

- I dont like being crammed on the same planet all the time

- I dont like the low variance of opponents either; theres only people, robots, and insects that attack you

- The combat mechanics are also not really that appealing

- All addons to the game increase the storytelling component, but that doesnt really interests me

But I can at least see how others may sink hundreds if not thousands of hours into RimWorld. With many other titles mentioned in this thread, this is frankly a mystery. Who plays Diablo endlessly ?!?