Eh, gee. I vote for "For Blue Skies", and did that entry really have to start with the word "For" so I had to put two "for"s in a row? 

In any case. I vote for it because it stunned me with exact proper amount of storytelling with a twist which was foreshadowed just enough to have been a superbly executed twist with a most ingenious use of a PS to confirm the suspicion and also have it as a fun and completely sensible way of putting it on the end. The way it was made as a personal letter made it feel so personal, and made the twist in the end much more difficult to realize, and despite the depressing nature of what had happened it filled me with a satisfied happy aura. I'll simply say that I enjoyed it the most, and therefore it gets my vote. Well executed, well conveyed and well written, I simply feel it rather perfectly did what it was written to do. That is my opinion in the end. That is all.

Edit: The runner-up for my vote would be Never Forget. Definitely.