Of course there cannot be just one, if for no other reason than that there's country blues and city blues. Let alone Delta, Piedmont, Chicago, Louisiana, Texas, Jazz, Boogie Woogie...well, you get the idea.
So just from the all the way through great, every cut, I'll list a few:
1. Muddy Waters - Best of Muddy Waters (Chess)
2. Howlin' Wolf - his 1962 releases: Howlin' Wolf & Moanin' in the Moonlight, now out as a 2 for 1
3. Robert Johnson - The Complete Recordings (or King of the Delta Blues)
4. John Lee Hooker - The Real Folk Blues
5. Albert King - Born Under A Bad Sign
6. Memphis Slim - Just Blues (Chess)
7. Lightnin' Hopkins/Sonny Terry - Last Night Blues
9. Buddy Guy - This is Buddy Guy (Live) (Vanguard 1968)
9. Rural Blues - V/A Folkways Records compiled by Sam Charters
10. Country Blues - V/A Folkways Records compiled by Sam Charters
And that's Just scratching the surface - B.B. King, Otis Rush, Elmore James, Sonny Boy Williamson, Willie McTell, Skip James, Son House and so many more all have many incredible cuts.
The first two listed are indispensable. You haven't received the transmission without 'em.