Mion is the most underrated singer in holo. Her range is amazing, her singing is clear and she's the only holo who was able to pull off something like Konayuki (which ranges from very low to very high), a song that even hard hitters like Towa (completely failed) and Watame (was reduced to singing it as an "opera" after failing).
Wolf hag is easily one of the best singing chuubas, despite being a part of Gamers.
https://youtu.be/SWuzFUJMEgMI'd swap Mori or Moona for her in a heartbeat.
Also, honorable mentions go to Towa and Kanata.