By default the plugins are ranked by “relevance” and the only people who know how that algorithm is weighted work for wordpress.org (it’s doubtful they are going to let anyone in on it).
Downloads and rating are not the main components, just one of many including keywords, date, and how many beers you buy Otto.
You don’t have any options to rank high outside using the right words to describe and tag your plugin and creating a good plugin that people like.