A few great places to start:
Muddy Waters -- Hard Again. This was a major comeback for him in the late 70s. It was produced by Johnny Winter, who also plays lead on the record. This album combines the hard, loud stomp of a South Side electric band with excellent, cleanly recorded sound. Muddy Mississippi Waters Live, recorded a few years later with most of the same musicians, is also stellar.
B.B. King -- Live at the Regal. This 1964 classic is considered by some to be the greatest live album ever recorded. It showcases King and his large band (full horn section included) in front of an adoring audience. His vocals and guitar are at their absolute peak, and his chemistry with the crowd is effortless. Every track is classic, but there are particularly stunning versions of "Sweet LIttle Angel," "Worry, Worry," and "Please Love Me."
You can't go wrong with this one. If you can track down a copy of the Mobile Fidelity gold-edition CD, you are in for a treat, since the beautifully restored sound matches the excellence of the performance.
Otis Rush - Right Place, Wrong Time. Rush is one of Clapton's main influences, and this legendary album catches him at his best. Includes "I Wonder Why," a signature instrumental, on which he displays a guitar tone that could melt steel.
Albert King -- Born Under a Bad Sign. Another huge Clapton influence. The title track, along with "Personal Manager" and "Crosscut Saw" are among the best electric blues ever recorded, period. King played a Gibson Flying V, and was famous for the way he used string bends and distortion to build tension in his meticulously constructed solos. The "Personal Manager" solo is definitive of his style.