I think I would like to nominate Purged of Sin's piece, A Teddy for Your Thoughts
. The category is Gundam 00 and the rating is 17+.
It's written so simply, but really provides the reader with an indepth insight of the world in a war-torn era. Even for me, someone who doesn't really understand much of the political aspects of the Gundam series', I found that it was quite an easy and enjoyable read.
Not to mention, it's very underrated despite how touching and beautiful it really is.