Browsing back over the monthly contests since I discovered FF, there are lots of stories that I remember fondly.

Narrowing them down to my top five:
[Mar 2014] "The Heir of Foulstania" by pornokitsch
[Jan 2014] "Stab and Twist" by ACSmyth
[Aug 2013] "A Fine Morning for Running" by ladygreen
[Jan 2013] "Podge" by blendyface
[Sep 2012] "Brother's Lament" by Lionwalker