Okay, when I said Marvin was the GOAT, I knew that was rash and I was typing too fast without thinking it through.
But your counter is kinda making my case.
the only person you mentioned who could take the title with ease is Stevie Wonder. When you compare Stevie's "golden period" (MUSIC OF MY MIND, TALKING BOOK, INNERVISIONS, FULFILLINGNESS' FIRST FINALE, SONGS IN THE KEY OF LIFE, the flawed but interesting SECRET LIFE OF PLANTS, and the return to form HOTTER THAN JULY) to Marvin's "golden period" (WHAT'S GOING ON, LET'S GET IT ON, I WANT YOU, the TROUBLE MAN soundtrack, the flawed but interesting HERE MY DEAR), Stevie's got more masterpieces under his belt.
Now that list doesn't count all their respective hits working in the Motown hit machine before they broke off and became creative auteurs.
R. Kelly's HAPPY PEOPLE album is great top to bottom, but it's him mimicking Marvin shamelessly. Are there some other masterpieces that he's made that I am not considering?
Curtis Mayfield is magnificent, with the SUPERFLY soundtrack, the SPARKLE soundtrack and the CLAUDINE soundtrack just to name a few great albums, but track for track his solo albums aren't as strong as Stevie's or Marvin's.
Smokey is the most underrated guy on your list, props for bringing him up, The TEARS OF A CLOWN album is a masterpiece, the QUIET STORM album is also perfect and launched a whole radio format, and his early works with the Temptations launched the great male vocal group of all time.
Your R. Kelly "argument" (as if a case could be made) is that he makes hits. Well, Stevie, Marvin, Smokey and Curtis have made more hits then R. Kelly. And Stevie, Marvin and Curtis inspired countless musicians and had huge cultural impact. R. Kelly doesn't have a WHAT'S GOING ON or SONGS IN THE KEY OF LIFE in his body of work.
As a musician, do you want to put R. Kelly's piano playing skills up against Marvin's?