I've said it before and I'll say it again: This is the best Pinkie/Dash story I've ever read, yes I love it even more than Butterscotch Sundae
's epic "The Party hasn't ended" albeit the two are close but "My First Party" edges in just a bit over "The Party Hasn't Ended". The reasons are fairly simple, while "The Party Hasn't Ended" is a great story the use of a supernatural entity to bring the two characters together (as a plot device I mean) sometimes makes me feel a bit odd, it's not a bad thing in this case but I've seen that plot device used to often that I've gotten a little tired of it, "My First Party" on the other hand opts to remove any supernatural elements from the story (well stuff that would be considered supernatural in a world of flying ponies and magical unicorns) and it becomes a fully character driven story that allows us to see how things progress naturally.
The biggest credit to this story is that it is so far the only well done Pinkie/Dash story I've found that isn't about Pinkie Pie and Dash falling in love, by setting up from the get-go that they are marefriends and in love we get to see something rare for this fandom: a story that explores a relationship while it's already in progress. With "My First Party" we only get a brief introduction that just tells us they are in love and thankfully the entire first chapter shows us just how much they love each other and this is what makes this story so awesome, it's not important how they fell in love, what's important now is how they handle now that they are in a relationship.
What really shines about this story and how it shows the characters is that Dash is willing to go outside of her comfort zone for Pinkie, she's not good with mushy stuff but she stays to cheer Pinkie up anyway and while it is a long trip she's there for Pinkie when they go to re-connect with her family. Dash is there for her and that's what counts, especially since we've seen both here and in the show that if she asn't doing this for somepony she loved then Dash might not be doing this at all
My First Party gets it where it counts, this story is all about mending broken bridges, reconnecting with things from your past that you thought you lost, and the love between Pinks and Dashie that makes it possible for Pinkie to face her inner demons and let go of her sadness.