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. ❤️ 🫡
Another answer for me is Super Smash Bros. Melee (but could probably apply to many multiplayer games).
Normally I’d always engaged with it in person, at events. But when I started playing online, it was almost too much of a good thing.
On the Internet, your next match is always 15 seconds away. You get beat, you go ‘I can do better, just give me another chance’. You win, you’re on a high, you don’t ever want to stop.
The game has that perfect push and pull (at least when you’re playing someone anywhere near your level) that just builds compulsion. You’re constantly engaged in decision making or quick button pressing each moment, and it just feels satisfying.