Hohoho, good question! Um.
If I have to pick one, I think it has to be Man Who Sold the World. Not because it’s my best (it probably isn’t), but because time travel holds a unique place in my heart (Doctor Who is my first television memory; I wasn’t yet four), and the things I’m doing with this story are things that I have wanted to do or experience in a time travel story for the greater part of my life. So I am…attached to MSW, yes. ^_^
In terms of a fic that did exactly what I wanted it to do, probably Anywhere You Go or my A-Team (movie) fic, Paradise by the Dashboard Lights.
I also have a special fondness for Stringplay, because it was my first real attempt at porn, and it cooperated with me so nicely.