Wow! You talk about a difficult choice to make! I picked Otis Redding but could have made a good argument for any of them as well as The Temptations (who, thankfully, never gave all of the credit to any one person, and had many great lead singers, David "My Girl" Ruffin being my favorite, but he wasn't even in the original lineup, and certainly didn't do all of the lead singing even in his era).
EDIT: Interestingly, with the sole exception of Jackie Wilson (who I never have quite been able to get into, though I've tried many times), when I'm listening to the music of ANY of the other mentioned names in this poll, I almost always get that chilled out, "It don't get any better than this" kind of feeling. With Al Green, it's just obvious, he melts you away. With Ray Charles, James Brown, or Stevie Wonder, its the genius that inspires.
With Curtis Mayfield (and also James Brown, I suppose) it's that cool, hip factor and the sense you get that they had a sense of exactly how to keep it original (it's always 'loose' but yet it's going somewhere that they know about before you do). Sam Cooke, Al Green and Marvin Gaye are probably the ultimate crooners in the list. For them, it's all about singing, and you can throw any song at them, it wouldn't matter. But Otis is special to me for some reason. Don't know why exactly, but it's the same kind of feeling I get when listening to Roy Orbison. It's like I know him from a prior life or something, but not as far back as when I was Cleopatra.