Pre-war for me has to be Robert Wilkins, who became the reverend Robert Wilkins, however I draw the line at the sixties rediscovery material which is weak. His 1927-1930 sessions are sublime.
My favorite electric vocal accompaniment is Sonnyboy Williamson (Rice Miller), however when he got discovered in the sixties and went to europe he made a series of terrible records which are actually quite popular. Lots of self-conscious tomfoolery and posturing for the kids. What I like are his chess sides, almost all of them, from the early 50's to about 1963.
Blues-rock is a difficult genre to choose within because of the astounding range of approaches. You really can't compare the best ones because they are so different. Pressed, I might choose Captain Beefheart (Do Van Vliet). Just listen to 'Mirror Man' from 1970. Genius.