Ahhh, fanfic. Where else can you read "new" episodes of your favorite tv show after it's been canceled or off the air for forty years? Where else can you "cast" your favorite classic actors in a 1960's tv show like Combat! or Voyage to the Bottom of the Sea or Bonanza and have fun imagining roles they might have played? Fanfic is a fantasy playground that produces some of the best fiction I've read -- and some of the absolute worst. It gives people a chance to author stories who might never have written a word of fiction in their lives otherwise, and gives them an encouraging audience to boot. Some of those go expand and write non fanfic stories, get published in the "real world." It brings fans together and has given me some of the best friends (and best vacations to get together with them) I've ever had.
Now, for a single fanfic? I'll go with my favorite Combat! story, The Weight, written by a talented lady nicknamed Ricochet. When I write fanfic, I mostly write as if I were a screenwriter in 1962 contributing an episode to the show. I fail to think outside that box, and that's why I love this story so much. It's outside the box, and yet still true to the characters as we got to know and love them. It works on so many levels and is just plain satisfying to read.