Tough to say. Johnson's domestic accomplishments with civil rights legislation and numerous domestic accomplishments
http://en.wikipedia.org/wiki/LBJ#Major_legislation_signed (also add establishing the National Endowment(s) for the Arts (and) Humanities) is impressive to say the least. But the impact of Vietnam can't be understated, and LBJ bares overwhelming personal responsibility for the vast escalation of American involvement.
Yes, Kennedy and Eisenhower had American troops there already, but
nowhere to the degree LBJ deployed after the Gulf of Tonkin. Also by most accounts (e.g. then Senate Majority Leader Mike Mansfield) JFK intended on gradually withdrawing the then small number of American troops in Vietnam after the 64 election. Anything could've happen, of course, but it seems likely we would've never had the Vietnam War if JFK had lived.
In all, tossup.