Since there's been a lot of discussion, here's some info about how signature performance works and two recent changes we made to it.
1. Until recently we were choosing a performance that might not have been on your best or your favorite instrument. Now it will be on the instrument you have the highest career (plus DLC) score on. (Keys/Pro-keys might be underrepresented because there are fewer songs that can add to their career score.)
2. We also recently started giving extra performance points for songs played on higher difficulties and for songs at higher difficulty tiers. Before, easier songs were too heavily represented in signature performances. This is called the "Charlene adjustment" because I didn't want Charlene to be my signature performance anymore.
Your specific rank # on a leaderboard doesn't affect signature performance because it's not normalized across songs. A rank of #1,000 on a leaderboard with 1,000 scores means something very different than #1,000 on a leaderboard with 100,000 scores. So it's not useful for comparing performance across different songs.
Stars and accuracy also aren't directly considered. What we do look at is your score, in a way that's normalized across songs. Scoring highly is the most important thing. Gold stars, 100% accuracy, and a high leaderboard rank will help because they go along with high scores, but they won't guarantee a signature performance.