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.
Void Stranger is a relatively recent one. It’s a Sokoban style puzzle game with layers of puzzles and a ton of hidden depth.
It took me 50 hours to feel like I beat the base game and I haven’t even touched the post-game content they added after release. I have a folder full of text files with notes and clues and puzzle attempts and one of the best puzzles involved taking several screenshots and stitching them together in an image editor.
La Mulana is another one to check out. It’s a metroidvania heavy on puzzles and exploration that’s actively hostile toward the player. It’s an exercise in frustration and every inch of progress is measured in blood. Every bit of information is important, and there’s a lot of information to untangle. I haven’t come close to beating it yet and my notes from just the first few floors are extensive.