NetHack! Been playing for 10+ years and haven't come close to finishing it yet. Fantastic game but a lot of work to get into.

And of course Go, but that's on a completely different level.

Factorio

Stellaris - Big real-time space strategy game

Dominions 5 - Fantasy turn based fantasy strategy game with tons of nations often based on real world mythology mixed with fantasy elements.

Kingdom Death: Monster - Tabletop (play on tabletop sim) game that mixes Darksouls/Monster Hunter like boss-fights with a little bit of Darkest Dungeons like colony management/attrition/madness

Bitburner

In it you write JavaScript to hack things to be able to write more JavaScript to hack more things. It's very fun. Especially if you don't know how to write any JavaScript.

100% Stellaris (assuming it counts)

RimWorld. Easy to play, hard to master. Also Rust

Shenzhen I/O and TIS-100 take some getting used to.

The Witness is amazing, if you like puzzle games that take a lot of time to figure out the rules.

Factorio for sure. It just so satisfying to build a well oiled factory.

Can also recommend Dyson Sphere Program, little less polish than Factorio imho but it's basically building production facilities spanning a universe.

These games are my goto for a "complex but still fun" mood:

• X series (X3 and X4)
• Endless Space 2, Endless Legend
• Crusader Kings 2
• Stellaris

I started Kerbal Space Program this weekend. It's pretty complicated.

Dwarf Fortress of course. Other good mentions are Factorio and Rimworld, if you understand them and are done with it, install a couple hundred mods and have a whole different game!

A minecraft modpack called Gregtech New Horizons. It takes the general modding scene of minecraft 1.7.10-1.12.2 era and pushes it to 11

Victoria 2. Haven't gotten around to trying the sequel yet, but will eventually. Been playing a bunch of Dwarf Fortress as of late too.

It's a mod, but the original long war mod for XCOM 2012 holds a very special place in my heart. You basically have to have reference material open because of how large the research trees got, along with just no ability to really in game teach you about any of the mechanical changes.

Maybe not quite as sprawling as something like a paradox game or dwarf fortress, but LW also demands you fully understand the complexities it introduces over the base game or you can find yourself in unwinnable or extremely difficult to pull out of positions, and you easily end up playing tons of missions in a doomed session if you miss, say, upgrading your interceptors enough quickly enough to pop the overseer UFO or find yourself in a massive resource pit because you sold all your elerium early and now you can't keep up techwise.

The sequel mod for xcom2 never to grabbed my attention the same way long war 1 did.

Europa Universalis 4, Crusader Kings and XCOM in harder difficulties in iron man mode. I love brooding over difficult decisions and devising strategies in turn-based (and similar) games.

Traditional fighting games, I suppose, namely older anime fighters/airdashers, at more than a super casual level. Among those, I find Guilty Gear XX Accent Core +R and BlazBlue: Central Fiction to be especially complex but also so, so rewarding when you put in the time to learn them.

Just to give you an example, this is a sort of basic combo for Kokonoe, the character I play in BlazBlue: https://yewtu.be/watch?v=U8j0mmLJ4zo&t=95 . I have been practicing for weeks and I'm still halfway through that.

Crusader Kings

Sounds like X4: Foundations might be right up your alley.

RimWorld and Cosmoteer.

Escape From Tarkov. I love/hate that game.

• X series - most complicated spacesims in existence (nothing realistic though)
• Stellaris
• Imperium Galactica 1 - a super-complicated 4x space DOS game - think Stellaris + SimCity + Command & Conquer
• An obscure unpopular old game - Battle Isle: the Andosia War - or as one reviewer roughly said: "they must have had a board where everyone who came up with an idea got a golden piggie - it would work as 3 games: Battle Isle, Economy Isle, and Energypipe Isle, but this is just too much". Basically a turn-based but real-time strategy game (unit movement is turn-based, you can end your turn at any time but... resource extraction is real-time and if you run out of energy and water simultaneously you can't cool your power plant and you can't power your pumps). Also maintaining a supply-line behind your advancing army.

I'd throw Hunt:Showdown up there. I started with a friend a couple years ago and we just devoured YouTube videos to learn tidbits. But I realize not everyone wants to watch people playing a game in order to learn and play a game themselves. I'm still constantly in awe of the sound design and how it meshes into finding intense gunfights and defines how you engage other players based on what guns you think they have based on sound versus what you know about your own loadout. Each fight I'm learning something new and I'm a little over 500 hours in. I think the "generally accepted" amount of time to get decent at the game is around 1000 hours.

Crusader Kings and EVE Online. I wish I still had the time to play EVE

I'm a massive fan of Paradox Interactive, and play almost all their stuff. Crusader Kings 3, Victoria 3, Hearts of Iron 4 and Stellaris (in that order).

The most complicated game I've played is Supreme Commander; the basic gameplay isn't too bad but if you want to fight anything more than an easy AI then oh my goodness it's tough

I don't know that it's my favorite, but I want to give Shadow Empire and honorable mention. Definitely not a smooth process for beginning it