This list is not arbitrary. It's based on a very scientific algorithm that you can try yourself here: http://kouhi.me/idol/
This took 311 rounds to decide my precise ranking of idols, so it's best to proceed when you have some time set aside for it.
1. Nishikino Maki (Love Live!)
2. Anastasia (Idolm@ster)
3. Shibuya Rin (Idolm@ster)
4. Minami Kotori (Love Live!)
5. Shimamura Uzuki (Idolm@ster)
6. Usami Nanako (Locodol)