recommend me some oft-sampled/"important" Soul/Funk/R&B artists whom you only have to own one or two records by to get a decent grasp of their work

Brenton Wood's 18 greatest hits
O'Jays Greatests Hits + Love Train: Best of the O'Jays
(one doesn't have Forever Mine and the other doesn't have I Love Music)

If you want to find some oft-sampled soul/funk/r&b, go here and then click on the soul, funk, r&b button at the top left of the page. It'll give you an alphabetical list of artists who are sampled, what songs are sampled and who sampled them.

James Brown: Live and the Apollo and Star Time. (Well, you didn't say anything about the 'Greatest Hits' being one CD.)

Nate, that thing's been remastered! "Star Time", I mean. Am totally lapping it up.

This thread's kinda useless, cuz I basically KNOW that all the major Soul artists that I haven't sought out (as in, knowing more than the hits) yet require extensive listening (hmmm, Curtis Mayfield box versus Smokey Robinson box...), and I'm in a spot where 1- I need to extend my knowledge a lot more and 2- I just don't have the time to focus extensively on a single artist. So I posted this, in the hope of having my cake and eating it too, but I kinda already know that's not possible. Oh well. (thanks anyway to everyone who suggested stuff.)

On a more serious note, then: Baby Huey's Living Legend.

There aren't so many artists of any kind that you can fully grasp with one album, unless they had a very short career - but there are decent best ofs, often available cheaply, for Al Green, Otis, Aretha, James Brown, Solomon Burke, Irma Thomas, Joe Tex, Etta James, Curtis Mayfield, James Carr and so on. The best example of someone great where one original album and a greatest hits works is Sly & the Family Stone, where their original greatest hits came out before their best album, There's A Riot Goin' On. I am not very clear as to why the frequency of their sampling matters much here, so I may be citing some people who are very rarely sampled (I can't think of any for Carr, Thomas or Tex for instance).

only need 2 discs:

curtis mayfield + the impressions (2 disc greatest hits)
aretha franklin (2cd best of + the 60s live album, maybe)
sly stone (greatest hits + "riot goin on")

counterexamples: al green and marvin gaye, for example.

etta james - "rocks the house" + "tell mama" or "at last"

everybody r&b fan should have these two (three) in the collection. yours, for less than $20!