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. ❤️ 🫡
Payday 2 probably takes the gold medal for me. I was once playing casually with a friend of me after work, chatting about the day, and when we were about to finish the level we noticed that we damn near broke the world record for that level, a record made by 4 players.
I only played payday 2 casually because all I would play is hoxton breakout LOL
I was invested enough though to make my own skin mod (pre lootbox) and it was kinda popular