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.