Komrade,
Most important would be make it a scenario that you're really interested in writing about. Even if you end up losing, if the game was, in your mind, well played or so fascinating that you can't help it, do that one. There's a newbie AAR thread, and all the advice you could want is really in that.
If the game was a challenge and interesting to you, that will show in your writing. So pick a country that fascinates you that way. And you might want to take a purview of the AARs in progress, and decide if one of the nation's being played right now is one of the one's on your list and being played the same way.
Note, if you're planning a serious AAR and the AAR of that nation going on right now is more comic, I wouldn't think twice about writing it still. But if you're doing the same nation the same way, you might want to think of something different or waiting for that one to finish.