Oh god, am I really about to admit this? Publicly?
Back when I was a teenager cutting my teeth on fanfiction, I wrote a silly little guide on how to recognize and avoid writing Mary Sues. This was just over ten years ago now. To this day, I still get people finding, reading, and reviewing it. It's been translated into two other languages, and one girl gave a small presentation on it at an anime convention (all with permission). Something I wrote halfway as a joke has survived completely on its own for a decade, and if the reviews are anything to go by, it's still helping people better themselves as writers. Honestly, it's the only reason I let myself get back into writing after I almost gave up. If something I did was good enough to earn its own legs, so to speak, maybe it was something I shouldn't give up on yet. It's my greatest writing achievement for both general popularity and keeping me going.