If you can find it, get the one in the picture by Richard Berry. That one seems to turn up the LEAST, in my experience, but it is out there.
Also, Johnny Otis had three albums in this series (why not, it was his damn label), and they're all consistently good. Some selections on the three Otis elpees are leftovers from his Kent years (the Cold Shot! album, Snatch & the Poontangs), but it is all highly listenable.
What I like about the series is how little the music has dated...any other producer would have tried to "update" the old hits with "fonky" 1970's arrangements, but the sound is actually quite spare for the time. This was at a time when the old West Coast jump-blues sound was all but forgotten (the Chicago shuffle thing was getting all the attention), and this stuff wasn't reissued as often as it would be later. So it was good for Johnny Otis to do this project. Hell, Amos Milburn was sick as a dog when he recorded his album in the series - and he sounds it - but it is still good in spite of that.
Related: Roots Of Rock by Roy Milton on Kent (later reissued on United), which I think was supposed to be the forerunner to this series, but only lasted for an album.