My picks. These represent what I’m inclined to listen to, and nothing more.
The Carter Family-- sang both Gospel and ballads. Their repertoire is the basis of much of what was to follow in Gospel, Country & Western, Folk, and even Rock and Roll. The oiginal Carter Family, who recorded 1920’s-40’s, had two female and one male singer. The later Carter Family (sometimes called the Carter Sisters) were an all female group-- originally Maybelle Carter of the Original Carter Family, and her daughters.
Chuck Wagon Gang-- it’s possible that a person needs to actually have grown up in church in rural America to appreciate their singing. I don’t think so, but I’ve heard they are an acquired taste. Sing in four-part harmony.
The Cherokee Children’s Choir-- sing a style of Gospel music closely related to shapenote hymnody, in the Cherokee language.
The Happy Goodmans-- some like them, and some hate them, but, again, they were without doubt very influential, until Howard and Vestal’s deaths in the past couple years. Vestal’s lead singing influenced a generation of female Gospel singers.
Cynthia Clawson-- a present-day vocalist, who sings a blend of traditional southern Gospel and more contemporary music.
The Kevin Nealon Family are a current family group whose vocal style I like.
Finally, one more current group bears mentioning, and that is the Martins. Like the Carter’s they are a family group, one male, and the rest female, I believe, and they sing in tight harmonies, sometimes a capella, and sometimes not.