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. ❤️ 🫡
Nothing will ever hook me as badly as Kerbal Space Program did. If I wasn't at work, I was playing Kerbal for five years straight. No breaks, didn't play anything else during that time. Once I got RealSolarSystem and RealismOverhaul working, you couldn't pry me away from the computer. I put in aver 10,000 hours, easily.