Mass Effect. I know some will disagree, the third game has a lot of glaring issues, and EA really fucked up the ending, but as far as a fully fleshed out story and universe with a multitude of unique and independently structured species, characters, and cultures I think it’s one of the best. The writing and possible story outcomes and decisions that vastly and permanently affect the story from the first to the third game are insane.

Try Yakuza

Neither of these are popular enough to be on the scale of LoTR, but in terms of atmosphere and detail:

Hollow Knight - my absolute favorite thing about it is each NPC has its own voiced language recorded, babbling in the background as you read the dialogue.

Subnautica (the first one) - shitting myself with each new experience is something I'll always cherish. Highly recommend just playing without looking into the gameplay or plot. Has elements of exploring, resource gathering, base building, psychological horror (not graphic, just tense scenarios), sneaking.

Clair obscur

Completely different genre but the Stabley Parable checks all of these.

100 years in the future and it will be just as perfect.

The first half of the description, combined with the unmentioned ambition of LotR, made me think of Star Citizen. But we'll have to wait another 5-10 years to find out if they manage to deliver on that ambition, and stand the test of time.

Enderal 🥰

It's not a trilogy, but I gotta preach the good word of Ender Lilies: Quietus of the Knights. If you enjoy 2D metroidvania style games it's top notch.

The game just drips in atmosphere. The environments are beautifully drawn and designed, characters and enemies are animated well and the music just wraps everything up in a nice bow. It's a melancholy game and it literally made me cry at points and I'm not the type to tear up often when playing games.

The game can be challenging at times, but I wouldn't say significantly so. I would say Hollow Knight is more challenging than this game especially with some of the end game content that that game has.

The game is worth full price, but it goes on sale pretty regularly and probably is right now with the summer sale on Steam.

There's also a sequel out now called Ender Magnolia. I haven't played it yet but I will eventually.

World of Warcraft, at least for the first 3 expansions. I think the people that made it back then were utterly devoted to the story and gameplay, as were the players. I'll remember playing that until I die, so many memories.

Downvote away.

Undertale and Deltarune.

Dwarf Fortress.

Ultima 1-7 and 8 partially.

Terraria! That game is great and aged like fine wine

FALLOUT NEW VEGAS! (Side note) Oh boy, a new list of games to play as I scroll through these comments.

A lot of good suggestions but my first thought was Final Fantasy X.

Ocarina of Time. I'm biased because of nostalgia, but I genuinely think it's the best game ever created. It took everything that was great about the SNES classic A Link To the Past, brought it into 3D as an early N64 game, and improved literally everything. The atmosphere, the gameplay, the story, the time mechanic, the music... It's not perfect, in fact these days it's trivial to break many things in it with glitches, but I think it's absolutely the best.

Hades

Prumbably the Morrowind game