I'm fairly new to the ocean of MLP fanworks, so when I decided to search for Derpy-oriented stories on a whim, Trails and Trials was among the first that I came across. This particular fic has shaped my expectations of what to look for in a good MLP story, and even weeks later, once I revisited it after reading plenty of other works, I feel confident when I say that this is still my favorite example of MLP fanfiction.
There are many Derpy fics that paint her as the perfect woobie. What separates this from many others I've read is that Derpy makes an earnest effort to not let her hardships cloud her being, and once given a little push forward by her fortunate meeting with Twilight, she stops at nothing to do what she can to change her situation entirely. Derpy's mission to learn the meaning of true friendship paints her as a hero of sorts for whom I found myself cheering the entire way. As a result, I feel like this particular incarnation of Derpy is one I can see fitting into the actual show very closely.
The writing is lovingly vivid and personal to the characters, and it's concise enough that it never embellishes itself with unnecessary detail. The author does a good job at portraying Derpy's environment and feelings via third-person, and the transitions to her thoughts are seamless and add much to the narrative rather than detract from it. The story's humor is entertaining as it often puts us in Derpy's mind, making little attempt to explain her surreal behavior and thought processes, but executes it well enough that the reader can accept it as being integral to Derpy's character instead of find it too absurd and out of place.
I do have a few issues however, particularly when Derpy is revealed to be illiterate. While I can accept this development since it gives Twilight another chance to guide Derpy with her expertise, it is neither alluded to previously nor expanded upon later beyond a few paragraphs in the chapter afterward. I feel like chapter 5 happened just a little too soon, and I would have loved to learn more about Derpy's reunion with her daughters. These are minor complaints, since the rest of the story doesn't suffer from such pacing issues.
Anyone can make a "heartwarming" fanfic, but this one is backed by empathetic writing and a conscious effort to keep true to the show's spirit and themes. I highly recommend it!