Call It Stormy Monday - Albert King, Stevie Ray Vaughan
These guys were just awesome musicians, they start with the song, then Albert announces to go up on A flat (0:39) SRV is like "alright", then right on the next bar they go on in A flat. They interact so harmoniously and intuitively, with joy and mutual respect, no show-off, just blues music at it's best. Really stunning.