Factorio, it's fun from the first minute but you'll still be learning new things after 1000 hours.

cracktorio

Per my steam library:

Factorio: 3,375.4 hours

Dyson Sphere Program: 2,505.9 h

Stellaris: 2,236.6 h

Terraria 2,629.9h

Skyrim: 1,239.5 h

Dungeon Defenders only has 600 hours on Steam, but I'm well over 2000 hours between Steam and PS3/4

I've also got a few thousand hours in Just Cause 2&3, as well as several Gran Turismo games and Forza Motorsport games. Morrowind probably has 2-3000 hours, oh and I'm not allowed near Sid Meier's Alpha Centauri anymore.

Secret of Mana and Chrono Trigger are probably up there as well since I'll replay them every decade or so.

Project Zomboid, Rimworl, Dwarf Fortress, Stellaris, Starsector...

Factorio

The Factory Must Grow

I’m almost 100 hours into my new space age save, and I’ve played nearly 1k hours since I bought it 5ish years ago. By far one of the most enjoyable games I’ve ever played and I highly recommend it to everyone who enjoys sandbox games

BeamNG.

I just can't stop crashing cars into things in slow motion.

The only two games I have that I've put more than 1000 hours in are Factorio and Rimworld. I'd highly recommend both.

Warframe. Inching closer to 3.000h currently.

It does come in waves, but every once in a while I go all in again and lose myself in the infinite things you can do.

I've been playing Tf2 for over 1.5k hours now. I recommend you to try it out There's a reason why that game from 2007 is still not dead.

Sims 3, Skyrim, Fallout 4

terraria for me. i keep managing to find new stuff over a thousand hours later.

"Has this item always been here?! no way. REALLY?"

Deep Rock Galactic

Factorio: 2344

Path of Exile: 2736

Rimworld: 2191

Kerbal Space Program: 1071

I have a bunch of honorable mentions in the several hundred hours ranges that are only not 1k+ because I have severe ADHD and something else became a hyperfocus before they hit that point:

Backpack Hero

Ark

The Last Spell

Timberborn

Factory Town

Dyson Sphere Program

Loop Hero

Brotato

Satisfactory

Path of Achra

Against the Storm

Desynced

Not a favourite anymore but still beats my second most played games by a factor of 4... Warframe was fun at the beginning and really clicked at about 200hrs. I left after about 2200hrs because I burned out but it still has a place in my heart.

There was a moment in my life when I thought that perhaps I was outgrowing games... that they were just not really designed for the entertainment of adult human beings in a way that could satisfy me. Not one of my old stand-bys could, as you said, "scratch the itch" or provide "the same enjoyment that it used to." I found myself spending more time with films and books.

Then I got into FromSoft games and indie titles, and within a year I realized games still held incredible experiences for me. I hadn't outgrown games. I had only outgrown the bland slop represented by most AAA releases, and especially by online multiplayer releases. Personally, I'd recommend giving up on looking for a new 1000+ hour obsession. Instead I'd recommend seeking a broad array of 10-100 hour loves. If you haven't already tried them, for me a love of gaming was saved primarily by: Dark Souls, FTL, Spelunky, and Hollow Knight.

Monster Hunter. The first one I played, MH4U back in the 3DS days, I put 1,000 hours into. That was nearly 10 years ago, and I'm still playing the franchise to this day. Currently finally going through the Sunbreak expansion of Monster Hunter Rise on the PC, and noticing a marked improvement in my mental health over playing other games.

I'm amazed nobody has mentioned Kerbal Space Program (first one). That and TF2 are my most played

Oxygen Not Included- very detailed colony sim where scientific things like heat exchange, air pressure, etc. are essential for survival. I swear, my next file will be perfect...

The Elder Scrolls Online was my first 1,000+ hour game, but it's since been long surpassed by Dead By Daylight.

Elite Dangerous. Well over 1,000 hours, especially with friends to explore the black with. Hard to get into, but it has so much stuff to do. It made me passionate about space! (And it's always fun reading articles about a far-flung star and thinking "hey! I've been there!")

Space engineers

Stellaris. I'm almost at 1400 hours in the game and while I now play it a lot less often than in the first 1000 hours, I still get the itch to play it again a few times a year.

noita

I'm closing in on 2000 hours, and it's such a great game if you like challenges and discovery.

I started playing it after one of the devs said, "I don't think anyone will ever make another game like it."

It's a terrific implementation of a very pure concept.

I really hope that, despite the development challenge it may present, "noitalike" becomes a thing.

I think it's an engine that would integrate really well with ML world/asset generation, too.

Satisfactory

Dota 2.

I've played Dwarf Fortress, Stellaris, yet I still find Dota to be the most complex game ever.

It sucks that it's a multiplayer game, and you need people to play it well with you, but when it works, it's amazing.

Everything in it has layers of usability, usually componded by the everything else in the game - hero abilities, items, map, neutrals, even the stupid trees play a large role in the game.

I've spent hundreds of hours studying the mechanics, and I still don't grasp everything.

I played a lot of DotA too, but that doesn't have a playtime counter. But I have over 3000 hours in Dota 2.

My second favourite game is OpenTTD. It's just so satisfying to optimize the train network and add another 100 trains to it. I've tried Factorio, but for some reason that did not scratch that itch.

Or some roguelites, like Slay the Spire (or the Touhou: Lost Branch of Legend), Synthetik (haven't tried second one yet), they are always fun.

Elite Dangerous is my go-to lately.

It's different to most other games, by not being goal-oriented except for the goals you set for yourself. No main quest line dictating progress. No mandatory tasks. No win condition. Instead, it drops you into a simulation of our entire galaxy roughly 1300 years in the future, where humanity has mastered hyperspace travel and spread through hundreds of star systems.

(To give an idea of the simulation's scope: Around 85 million systems have been recorded by players so far, and those are a vanishingly small fraction of what's out there. Space is big.)

I like that it offers a variety of activities to fit whatever mood I might be in on a given day. I can hunt pirates, mine asteroids, engage in a bit of piracy myself, find and collect bio samples, infiltrate rival settlements, venture into vast unexplored areas of space, discover Earth-like worlds that nobody has ever encountered before, defend humanity against hostile forces, photograph beautiful stellar phenomena, rescue stranded survivors, customize and finely tune my ship to perform beyond its original specs, team up with friends, pledge to a political power and expand their influence, or chill out as a space trucker and haul cargo to earn enough money for my next upgrade. It can occupy all my attention, or just be relaxing entertainment while I listen to music or an audiobook.

It's an MMO in the sense of having a large game world (galaxy) shared by all players in real time, but PvP is optional. One mode exposes you to other players, while another limits you to NPC encounters. You can switch between them at will.

One warning: A space ship has more than a few controls to learn, and they're better suited to a game controller or HOTAS than a keyboard and mouse. I use button combinations for almost everything beyond basic flight controls, since there aren't enough buttons on a controller for everything.

No Man's Sky. A foot deep but a mile wide.

Project Zomboid, Valheim

I am Not even close to 1000h but devinetively Factorio. Be aware, this game is a massive time sink and makes you play for way longer than you want. Especially with the new expansion you can spend enormous amounts of time in this game.

Pavlov VR

Path of Exile. Hands down.

I just broke 1400 hours and still going strong. There's so much to do and so much to learn, and it's so good at rewarding grinding and keeping you chasing those incremental improvements. It's 100% replaced RuneScape for me.

I have broken 1000 hours with Cookie Clicker, Guild Wars 2, RuneScape, and Eve Online. I don't recommend the latter two anymore, but CC and GW2 still hold up.

Honorable mention to Factorio. I'm still in the hundreds but it's climbing.

The only two games I have that much time in are Factorio and Satisfactory.

Dark age of Camelot. 6000 hours on my main character. I prefer not to think about how could life be different now if I had not started this game…

Minecraft...

Ark.

Mount & Blade II: Bannerlord with some mods took long enough to give up for me.

Garry's Mod. Basically a gateway drug to hobby animation, and in some ways not so far off from the modding tools used to make it.

If you've watched stuff like Heavy Is Dead, they're usually made with it. Some more professional-looking stuff is instead made in "Source Filmmaker".