There is two ways to get your replay there.
1. (Not completely sure, just based on a rumor)
Staff check the each attachment attached on the forum, and if they see a replay truly incredible, they'll make it a default replay.
2. There is also nabi sp competitions, in which you post 2 replays for a huge prize plus a chance to get your replay on the default replays folder.
It isn't annual, though.