For me it was Terraria, Satisfactory, Tetris Effect Master Mode, Minecraft with Mekanism mod & CSGO / CS2 (almost 2000 hours in CS alone).
Edit: There are so many answers that I might only reply if I have something interesting to say. I will read all of your answers though. 🙂
Edit 2: It has become too much for me and I have to disable notifications. Thank you for the massive response and please continue to discuss with everyone here. ❤️ 🫡
So many, it’s generally how it goes for me as I mostly play one game at a time. But some that have REALLY made me dive deep and play many hours a day for long times: Halo 1, 2, 3 and Reach, Elder Scrolls Morrowind, Oblivion and Skyrim, Minecraft, Terraria, Starbound, Satisfactory, Forza Horizon 4 and 5, Rocket League. There are many more, these are just what I thought of at the moment.
And the one I still play a ton and see no end in sight is Warframe, such a good game, and both the community and the devs are awesome so even after getting 1,200 hours in two years I’m still giddy about it and have so much fun.