http://www.fimfiction.net/story/38381/pinkie-pie-discovers-coffee
The tale of what happens when the most hyperactive Pony in Ponyville is introduced to the Equestrian equivalent of Starbucks. Chaos ensues!

At the time I found this fic, I was certain nothing could top it in terms of straight up, over-the-top Pinkie Pie randomness. But as you'll see with the other two Pinkie Pie fanfics I'll be uploading, I was quickly proven wrong.