There is a good compilation (cheap too) from the Columbia series called, Legends of the Blues, Vol. 1. Among its virtues, it has one of the best versions of Son House's "Death Letter Blues" -- to my mind still the peak, though Robert Johnson fans might disagree. I think in many ways the first volume of Leadbelly recordings (by John Lomax) while he was still in prison is a must have, but there are different versions of "must haves". Many of the Folkways albums by John and Alan Lomax are basic documents: you don't really know the period unless you have listened to them. But do you want must-haves to get a picture of the period, or do you want to have must-haves that you will listen to over and over again for pleasure? For instance, if you want to have a picture of the period, the "Masters of the Delta Blues (Friends of Charlie Patton" as already noted is a must -- but I don't listen to parts of it much. I also confess to not listening much to Charlie Patton except as an archive (this is heresy). It also depends if you are trying to learn how to play blues -- then of course, you can listen to anything for hours on end. But Son House, Robert Johnson, Big Bill, Muddy Waters -- you can (well, I can) listen to them endlessly. Yours, Peter