I'd say that he was not only the most influential economist, but that he also had the greatest benefit to society of any economist of the 20th century. His contributions include (but are not limited to): taming inflation, introducing floating exchange rates (vital to reducing financial crises), ending the draft, the earned income tax credit (or negative income tax, to use his words), a better understanding of people's spending decisions, school vouchers, the importance of free AND competitive markets, the close connection between economic and personal freedom, and the lifelong economic truism 'there is no such thing as a free lunch.'
Moreoever, he was absolutely lucid and brilliant until the end. I had the great pleasure of hearing him give a talk in 2003. Without a doubt, that is one of the things I'll never forget.
"Teach children that they have great potential because they are human." -Warrior