that's hard to say. Dumas Malone's Jefferson and His Time is one of the more extensive biographies of Jefferson. It is 6 volumes, and covers pretty much all of Jefferson's life. Malone won a Pulitzer for the 5th volume of the bio. I generally prefer details over summary in history books/biographies, and Malone certainly provides that. However, the final volume in the bio was published in 1981, and at that time historians could not conclusively determine whether the 180-year-old rumors about Hemings and Jefferson were facts, or just rumors started by detractors. Annette Gordon-Reed's book about Jefferson and Hemmings (published in 1997), and subsequent DNA research was able to conclusively prove that Jefferson was the father of Hemings' children (well, those that had descendants).
To expand on your questions a bit, Jefferson is one of the most complex Founding Fathers. As the primary author of the Declaration of Independence, he's on a pedestal with only a few other Founders. At the same time, it's impossible to synchronize the author of such powerful words with a number of the actions Jefferson took in his private life, as well as actions he took as President and as founder and leader of the Democratic-Republican party. In that context, I'd be hard pressed to call any biography of Jefferson the most authoritative. The top biographers of Jefferson, before the Hemmings issue was resolved, were Malone, Merrill Peterson, and Julian Boyd. Each wrote excellent books about Jefferson.
As for more recent bios and history books that focused on Jefferson, I've enjoyed the following:
Thomas Jefferson: The Art of Power, by Jon Meecham
An Artist in Treason, by Andro Linklater
American Sphinx: The Character of Thomas Jefferson, by Joseph Ellis
Setting the World Ablaze: Washington, Adams, Jefferson and the American Revolution, by John Ferling
Madison and Jefferson, by Andrew Bursting and Nancy Isenberg
The Great Divide: The Conflict Between Washington and Jefferson That Defined A Nation, by Thomas Fleming
Each of the above are good books, but I'd probably pick Fleming's book, Ellis' book, and Meecham's book as the best of the list. There are also a few others on my shelves that I've obtained in recent years that I just haven't gotten to.
BTW, in checking my shelves, I did notice that last year a relative gave me a used copy of Fawn Brodie's bio of Jefferson. I haven't read that in decades -- I may have to re-read it in the near future.
That a big investment of time. I'd suggest reading one of them and deciding if it was worth it to read the other 5. If it were me, I'd probably opt to read a few of the books I listed above, as opposed to reading all 6 volumes of Malone's biography.
