One of my all-time favorites: The Reverend Al Green, and my favorite song of his (even though they're all damn good):
Love and Happiness
I second the previous recommendation of Solomon Burke. One of my favorite Solomon Burke songs:
None of Us Are Free (can't find a good version of his on youtube)
Another favorite of mine is Marva Whitney. She's categorized more as funk but then funk and soul are pretty much sister genres, so I'll include her.
Marva Whitney - Things Got to Get Better (who's that bad-ass cat in the shadows directing the band...)