Some great choices here already. A couple of observations: Aretha Franklin is cruelly under-rated by the 'soul scene' if you ask me; forget many of the more well-known hit songs (many of which are fantastic), listen to material like "I'm Your Speed" from Almighty Fire and tell me this woman cannot sing. Also, I think for the first four or five years at least of her Atlantic career, the label steered her away from pop excess and cut her on material which deliberately and tastefully showcased both her incredible voice and unique piano playing. They were in it for the long haul, and made tons of amazing records.
Brenda Lee Eager is a simply sensational singer. I've raved about her on here before. Along with Shirley Brown her output provides a masterclass in terms of soul-singing in a 'technical' sense (if there can be such a thing).
Bettye Swann had a wonderfully expressive voice. I feel that she was let down by her material quite often, but when she sang truly great songs like (her own composition) "My Heart Is Closed For The Season" on Capitol, George Jackson's "Living A Lie" on Fame, or "Time To Say Goodbye" on Atlantic, she was an awe-inspiring vocalist.
Lolleata Holloway has few peers when it comes to power: check out "I'll Be Gone" from the Cry To Me album or the wonderful, unissued (until a 100 Club Anniversary 45) "This Man's Arms": spellbinding.
In terms of conveying emotion, Denise LaSalle at her best takes some catching and made some phenomenal records, a comment which applies to Candi Staton, Sugar Pie DeSanto and Maxine Brown.
A singer I must mention as a personal favourite is Kim Tolliver. She made numerous super 45s, but for me her foremost moment on wax is "I'm Losing The Feeling" from her Chess album: this must be in the best three records I've ever heard, and is a shattering, genre-defining example of SOUL.