God, what a great question!
there are so many that I love... I have to go with Mel though, It isn't my favorite fic necessarily, but I think Shades of Grey would make the best movie.
I think it's the most cinematic and has one of the most clear cut story arcs and character development with more in mind than just getting Jack and Ennis together. Also, I think she has some really well written OCs with Madrigal and Colin (and Heinstroza especially) and I would love to speculate on who to cast... (we all know Christian Bale is who's his face
)
Though, I would love to see Nobody's Angel made into a movie, too...
And if the Pirate fic ever got finished
And probably anything by Bailey could be transported into a movie... Line Squall? Just Another New Kid in Town? Boot Hill? All of those would be great...
Ok, I apparently have a problem with the "one" part of this question