I find it too difficult to pick one over another. It depends on what I'm listening to at the time. That said, I probably lean more towards blues guitarists than just singers.
My favourites are probably much the same as most peoples, Buddy Guy, B.B. King, Albert King, Albert Collins,etc,.
I think Stevie Ray Vaughan had the best Rock/Blues sound and Robben Ford plays the most amazing Jazz/Blues licks.
In terms of jamming with other musicians, the blues is a great vehicle because it's usually a simple structure that any level of player can join in with and is great, knock-about fun wether you can spring dazzling solos or just a couple of B.B.Kings well chosen notes.