LBJ continued JFK's legacy insofar as Civil Rights, but not with respect to Viet Nam.LBJ gave us civil rights and many other great programs. And I would stack his wit up against any post war President.
Overall I think with two notable exceptions I think the US has been blessed with pretty good executive leadership post WWII.
The Warren Commission which he established, was a white wash, and not a true investigation of the truth. He forced Earl Warren to partake (as with other reluctant persons) with the threat of nuclear war if word got out that it could've been the Soviets or the Cubans behind the assassination.
LBJ was no dummy that's for sure. But he wasn't a saint either, and probably worse than any Kennedy if one considers the scandals behind the scenes (eg. Bobby Baker).