I think is definitely Lazy, by Deep Purple. You may think that's funny at first, the concept of Deep Purple writing the greatest blues song ever, but if you listen to it you'll see that everything they're doing in the song is standard blues. To me, I consider Lazy almost like an expanded interpretation of Steppin' Out, as done by John Mayall and the Bluesbreakers. Lazy to me is almost like the son of Steppin' Out, they're undeniably linked together in my brain.
Not only are the riffs really similar, but Ian Gillian almost echoes John Mayall's voice on the track, it's absolutely unreal. The guitar is so bluesy and heavy in both songs, but Jon Lord's organ is so reminiscent of Booker T's. Definitely the greatest blues ever written.
Here's Steppin' Out:
