Favorite so far is definitely my biggest, "Kaede's Rhapsody." I've greatly enjoyed writing it, and it contains plenty of the best fiction work I've ever done. The blend of romance, porn, comedy, drama, and character work I've been honing on it can be exhausting to maintain across chapters. That, combined with constant tweaks to the outline for better pacing and structure, have made this project much bigger, and made it take much longer to finish, than expected. Even so, I'm still having fun, and I'm glad readers get a kick out of it too.
As for underrated, some of my Femslash February one-shots are definitely more popular than others. I'm grateful that anyone has read any of them. That said ... perhaps more folks might want to read "Ghost in the Machine," my sakuraoi fic with a tragic twist? Or "Scene from 'The Tragedie of Sato and Natsumi'," which shows another side of the Twilight Syndrome Murder in Shakespearean prose style? I take pride in everything I write, but these two stands out to me.