Johnson more liberal than Clinton who is more liberal than Obama?
It's hard to make these comparisons across time. Too much changes from age to age. Johnson's era was still the age of big government. Liberal plans and projects were less ambitious in the Clinton years. So in a sense, one could say - and many people have - that Johnson (or even Nixon) was the most liberal president. Also, Johnson had a Democratic majority in Congress throughout his administration - a very large one after he was reelected - so he was able to fulfill more of the liberal agenda of his day than Clinton or Obama.
The "Greatest Generation" lived in an age when big government was accepted and people had much faith and great hopes when it came to government action. Later on, people were more skeptical. Those of us who came along later might not have been so different from the WWII generation, if we'd had their experiences.