Best singer? Prince.
Best band? This tough.. oh, I would have to say it’s between.... Prince & The Revolution or Prince and The New Power Generation.
Wrote, composed and produced all his music. Crossed over into all major genres and had an impact in them, has an endless number of timeless hits, had a huge vocal range, created and managed multiple bands (wrote and produced for them), could play multiple instruments (played all 27 on his first album and most on all others), nothing compares to him.
When I say wrote and produced, I don’t mean he purchased co writing/co producing credits like pretty much all ‘musicians’ today, he actually did it (by himself most the time but there are some exceptions).