Stargazing: Koi no Signal Rin Rin Rin! - Ida Riho
Something about Hoshizora Rin put a big smile on my face from the get-go. While I'm not a fan of the trope of adding the "nya" suffix to everything you say, I always liked her peppy energy and her dedication to help her best friend Hanayo at the expense of her own happiness at times.
She also is given a fun voice with the help of seiyu Ida Riho, and this character song is so fitting for Rin. It even has her name 3 times in the title! It also takes quite a different tone to the other traditional idol songs sung as a group within the show itself, which was refreshing to hear.