A really, really good one. One that has interesting plot twists, and red herrings, and trees growing out of the library of a castle. It should also have compelling characters and a hint of irony. Avoid flashbacks, and flashfowards. Especially avoid flashsidewayses.
You could write a story about an elephant with amnesia, or a frog who thinks he's a toad, but Microsoft might sue you for those, so I'd avoid them.
Really the answer depends on the contest. If it's a furry story, maybe something cross-cultural. If it's super-heroes, maybe the super-hero doesn't WANT to be a hero? Maybe he's afraid of failure, or even more afraid of success? Maybe he doesn't like being the center of attention. Or you could write about the duality of mankind. The villain isn't actually such a bad guy, and the hero can be a jerk when he's not off saving the world.
Or you could write about time-travelling werewolves from the future, who come back to our time to prevent a tragedy that would result in the end of their race. Only in doing so, they set in motion a chain of events that may result in something far, far worse, only hinted at in the first comic, and brought to light in future issues. Yeah. Write about that.
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