I personally believe Billy is the most underrated player ever. He was tapping before anyone knew who Eddie Van Halen was. Brian May admits to "knicking a bit off Billy." Hendrix was even quoted on live television as saying Billy was America's best guitarist.
He is a monster blues-rock guitarist in terms of technique & style. He has influenced players for going on 40 years now. However, regardless of what you think of his playing, his signature tone is unparalled. He is one of the few guitarists that I can recognize almost instantly.
In terms of known songs, have you really listened to the solos in "Sharp Dressed Man?" Guitar World ranks it #43 on the all-time list. I'd vote "LaGrange" as another one of the "hits" where he amazes.
Some of my faves cuts for his guitar are "Jesus Just Left Chicago" & "Beer Drinkers & Hellraisers" (check out the hammer-ons).
I found this discussion which will help:
http://acapella.harmony-central.com/archive/index.php/t-1607006.html