Hard to decide since there are many good ones. I think of:
Hikari no mukou e / HACHI
https://www.youtube.com/watch?v=a117kgda0swSasame Hoshi / Kagura Nana (full version unavailable on youtube, and in fact you can't legally obtain it)
https://www.youtube.com/watch?v=aPeqTnRR_XE&t=43sStarPainter / Yuki Michiru
https://youtu.be/7mmWezN0nBoStep outside hololive and there are god-tier singers like Leona and HACHI (who are frankly too good for this rabbit hole). But this is about best songs instead of best singers, so I also include Nana and Michiru's songs.
