Who is the best singer/songwriter of all time and why?

I'd put Burt Bacharach and Carole King on the short list.

I wouldn't know about "of all time" but in modern time I would have to go with either John Lennon or Paul McCartney.  Take your pick.

According to me, Freddie Mercury. There were so many things he could do with his voice, and he a great way of arranging music. His songs are almost addictive. You can keep listening to them without getting bored, almost as if you notice some new arrangement in them the next time you hear it. His style and sheer voice quality allow me to say that he was not only the best, but ahead of our time as well as he was his own.

Impossible to answer...(but fun to try!).... John Lennon Richard Thompson Carole King George Gershwin I could go on.....

I would start with Freddie Mercury... Look at his influence on Art Rock...the blending of orchestral music with glam rock... Then I would have to mention Tom T. Hall. 11 #1 hits, 26 more top 10, he is known as "The Storyteller". Following him would be a list including Madonna, Michael Jackson, Barry Manilow, David Foster, Roger Miller, Jackson Browne, Kurt Cobain...these folks changed their genres in immeasurable ways.