Wow, interesting.
Six more back catalogues to explore!
Can you guys suggest some albums
to start out with?

Hmm...
I *would* say Millie Jackson, but she never effectively
reached that 'really good' status for me. More of a bawdy novelty.
Dolly Parton? The Bargain Store? Mule Skinner's Blues
I dunno seems like i'm getting kinda campy...

Hm...I just heard a Traffic album that was amazing!

i think there was a mixed bag of stuff from the "divas."

part of it is probably that someone else was generally producing their music looking for "hits."

gladys... i think if she worked with the right person there's still a lot of potential there--it's not like her voice went to pot, just the tunes.
diana... never thought she had much skillwize, but "diana" (produced by nile rogers) was pretty good--mostly about the tunes, not the vox.
aretha... see gladys, but add "who's zoomin' who" for at least a couple singles (the title cut, "freeway of love" and ?)
tina... her cover of "i can't stand the rain" is cool (i still prefer the original) but i have to say imo her best work was with ike.
chaka... yes, her best work is really with rufus but... that's like saying coltrane's best work was with miles--there's so much OTHER stuff that is great. i don't think chaka fell off till the 90s when she had lousy soul stuff on offer and TERRIBLE house guest shots. i wish i could afford to work with her.

recommendations for ben:

gladys knight & the pips from the 70s (titles on buddha are preferable to motown, but most of it is good.)
diana ross, my opinion is "diana" is the best thing she ever did. if you like "girl groups" you might want to check out the supremes.
aretha... most things on atlantic are good. i'm a fan of gospel so i recommend "amazing grace" (a live recording) but i'm biased.
tina... best of ike & tina (best selection for a beginner.)
chaka... definitely get the stuff w/rufus (i.e., rufus feat. chaka khan--they did a couple of records w/o her.) but i also like some of her solo stuff from the 80s.