Arthur M. Schlesinger, Jr. , 1917-2007
Schlesinger, Jr. was known by the general public primarily for his staunch liberalism and his work in the Kennedy White House, along with such famous books as A Thousand Days: John F. Kennedy in the White House. Graduate students of my generation, though, knew him not as a famous historian of the recent past, but as the historian who first shaped our understanding of Jacksonian America with his marvelous first book, The Age of Jackson (1945). Though this book has been much revised (and sometimes much maligned), it is still the starting point for any scholar who wishes to understand Jacksonian America. It is also, aside from its scholarly value, a beautifully written book.
I also think Arthur M. Schlesinger, Jr. can still serve as a model for how historians can do excellent scholarship but still write for a broader audience and engage in public service. I can't think of any historians now who serve in a similar capacity.