It would have to be Curtis Mayfield for me. I bought a soul pack in 1970 which had Sometimes I Wonder in it. I loved it and didn't know it was meant to be a b side. I picked up a couple more Impressions singles on Shude Hill market for pennies. A bit later Move On Up came out and I bought it as a new release. Soon after I found a copy of Big Sixteen and that was me utterly hooked. He was responsible for so much wonderful music through his writing and work on productions for Okeh, then all the glorious stuff on Curtom. His influence on Soul music was immense. My absolute soul hero.
In the 70's and 80's Curtis played live in Manchester very regularly and I saw him many times. He was brilliant every time.
Honourable mentions for Gil Scott Heron and Bobby Womack.
If I were allowed a shout for a female artist as well, it'd be a tough, tough choice between Aretha, Gladys or Brenda Holloway. So I'm not choosing.