It took me awhile to think of my two favorites.
Ants at a Picnic a sweet little story that got the Visitor series started. It was one of my first fics and had most of the main characters. Read it again always makes me laugh!
On the other end of the spectrum is my longest story
Airport clocking in at over two hundred thousand words... yikes. Now that was a monster, but oh so much love went into writing it.