Skyrim, Fallout 4, Sims 4 and Stardew Valley. I have only actually gotten Stardew Valley to work flawlessly... The other 3 I have failed to get working. Just recently this year, I tried manually making my own mod list for Skyrim worth around 400gbs all for it to just crash on start up haha. I spent a week doing that and when I hit that wall, it was such a grueling experience. Personally for me and in my cases, it's never a mod's fault. It's my fault as a modder. One of these days I'll build my perfect Skyrim, one day!
First one was probably a simple decensor mod for Sims 1. I've also done Stardew Valley and Skyrim. Probably a few more where the NSFW content was just some skins
Skyrim, and I don't think I'll ever go back to modding. It's just so much wasted time looking up mods, learning how they work (Bodyslide and stuff), resolving compatibility issues, going in-game, finding more compatibility issues, changing my mind about some mods, uninstall some & install others, more compatibility issues… I must've spent at least twice as much time on technical stuff as I did actually playing the game, and even then I could never quite get it how I wanted. For instance, my graphics card is pretty good (or it was when I bought it), but I could never get EMB, anti-aliasing, or shadows to work correctly.