For acoustic blues, at the top of my heap is Muddy Waters - Folk Singer on Mobile Fidelity Sound Lab. Buddy Guy - Sweet Tea is also really good.
On the electric blues side Some of my favorites include:
Robert Lucas - Luke and the Locomotives. AudioQuest put out some really well recorded blues albums.
Eva Cassidy - Live at Blues Alley.
Albert King - Born Under A Bad Sign (MFSL).
Albert King with Stevie Ray Vaught - In Session
BB King - Live at the Regal (MFSL).
Terry Evans - Putting' It Down and Blues For Thought
Mighty Sam McClain also have some great titles.
Howlin' Wolf - The Real Folk Blues (MFSL)
John Lee Hooker - The Healer (MFSL)
Junior Kimbrough - All Night Long
Otis Rush - Ain't Enough Comin' In
Shelby Lynne - I am Shelby Lynne
Luther Allison - Soul Fix'n Man
Lots of great music out there