Interesting question, regarding current artists not many would (could??) sing in that style, think there are a few with the ability. Mary J Blige for one I think could cut it live, but her material may not always lend to that, I have a cd by Denetria Champ who I think vocally is very similar but its a gospel cd so not sure you can compare directly.
For old school its even more interesting, do people think Aretha Franklin is the most soulful? She is probably my no. 1 (so much so for my 40th last year we travelled to cape cod to see her "farewell concerts as was desperate to see her at least once in my life) but there are others who could qualify for the most soulful but were not successful.
Loleatta Holloway was mentioned, Rozetta Johnson is another in similar style, there are many more who for a one off could equal Aretha. But overall she is my no. 1, even on some of her more well known stuff she does it for me, think I might have an Aretha day now!!
Cheers
Jock