Applejack, Rainbow Dash and Rarity climbing the Archback Mountains from "It's a dangerous Business, Going Out Your Door".
This fanfic has a special place in my heart as it was the first fic I have read. I didn't think self-published literature could compete with other fiction and have never tried it. It had some glowing recommendation though so I decided to try it. I did not realize it was a middle earth crossover until I was well into it, causing some "this seems familiar" thoughts on the way. I loved the LOTR and The Hobbit(Movie coming out soon!) when I was young(er) and it was quite nice to revisit the world somewhat. I was quite pleased to have my prejudice against fanfics removed.
Not only did it prompt me to look at other pony fanfics after I finished, getting me to gems such as FO:E and others, it got me wanting to draw as there was relatively little fan art of it at the time. (My first ever art I have done in years was of it. I have not posted it on here, it was really bad) And needless to say I love both of those things now.
So thank you Jetfire, it might be a little overboard to say you changed my life but, what the hell, you can say it anyways.