As an RPer, my favorite goes to Luster Academy. Not because I was particularly talented at the time I took part in this RP in 2007-2009, but because it was a long-standing, ongoing, RP where we (me and my group at the time) created a full history of a world going from moderately powered martial arts shenanigans to interdimensional conflicts where the destruction of planets were an after-thought. Essentially pantomiming Akira Toriyama's Dragon Ball from start to Super before Super was even a thought.
As a writer, my favorite goes to Mildfire, a sadly now deleted story I'd written which was intended to be a parody of all the (in my eye) worst aspects of YA fiction, where the protagonist is a chosen one against her will, part of an unlikely love triangle with two supernatural entities far out of her wallflower league, but instead of going through the tale with this status quo in mind; immediately self-reflects, refuses the call, puts the role of chosen one on the shoulders of her much more competent best friend who always wanted to be someone special, and starts working on herself to actually be a worthwhile love interest to her two potential partners. Through this she ultimately realizes that she is much more suited for one than the other without having to go through the dramatics of going back and forth between the two for three books when we all know the first one she met was going to be her final choice anyway, and in the end she doesn't save the world by herself as a barely adult teenaged girl, but instead convinces her friend the chosen one to maybe refer to the figure of authority and powerful wizard who has been their mentor all along and convince her (the mentor) to hide in the shadows and cheap shot the big bad in the middle of their duel because screw heroic sacrifices. The story then proceeds to skip forward ten years to our protagonist living alone in a one bedroom apartment in the downtown of some random city, revealing the whole story was actually her journal written from her own point of view, and receives a text from the second love interest inviting her out for drinks, which she agrees to. Turns out—she realizes while looking at the tan mark around her ring finger indicating a recent divorce—that as beautiful as her angel partner may have been; being with a partner who is physically incapable of committing any kind of sin leads to an extremely disappointing sex life.
...unfortunately no publisher was willing to take it up, and I didn't think to back it up before my previous PC died, so that story's fully written out version is gone forever now.