Favourite story on GA? Wow, now this is a hard question. There are so many amazing stories on here.
I however have to settle on The Rider's Pride by DKStories. Something about it just moved me so much. The story is a fan fiction of Pern, which I haven't actually read I admit, but I just adored it. It is set in a scene of inevitability, a world doomed to be forever under attack. The main character is one of the many who fight to keep the equilibrium, breaking out of the traditional work of their father because that is their dream. Ultimately, he finds comrades, the love of his life and fights as he wanted to. Yet now it is no longer the shiny dream of war but the cruel reality.
The story ends on a note that left me balling, because in some ways it was so perfect, and yet in others so tragic.