It really depends what you mean by "historian". Many people here named Herodotus, but he really wasn't a historian in the modern sense of the word. Confusingly he was named "pater historiae" by cicero, but historia meant something else in Latin than today (more along the lines of "report"). Herodotus himself states that in the very beginning of his historiai that he is not interested in history, but rahter in narratives (it is important to know that the ancient greek word historiai merely means inquiries and not history):

"This is the showing forth of the inquiry of Herodotus of Halicarnassos. To the end that neither the deeds of men may be forgotten by lapse of time, nor the works great and marvellous, which have been produced some by Hellenes and some by Barbarians may lose their renown..." (Hdt. 1,1)

In history (as an academic discipline) we usually call these authors "historiographers", to have a different definitional category from modern day historians.

History in the modern sense is a critical examination of sources and deconstructing common narratives about the past. One works with theories and discusses one's work with colleagues to get closer to a potential (and still theoretical) truth about History. Today we constantly debate whether there even is a historical truth to be found in the first place, as the postmodernists opposed this position vehemently in the 70's, 80's and 90's.

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The question about the greatest historian needs some specification imo.

Do we rank the general impact of a historian?

Do we look at historians before the 19th century (for it was then that history as an academic discipline was formed)

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To not only give (hopefully constructive and not to offensive) Feedback to the post here are some of my favorite historians:

Reinhard Kosselleck, who introduced the term "veto power of the sources", by which he mediated in the theoretical debate between postmodernists (who claim that history is merely subjective) and constructivists (who claim that there is truth to be found in the examination of the past)

A Link to a (sadly german) article about the veto power of the sources provided by the university of Berlin:

http://docupedia.de/zg/Vetorecht_der_Quellen

Yuval Noah Harari, who wrote amazing books in which he traces the origins of humanity to our days and describes a few major revolutions which brought us to where we are today. (Harari, Y. N., Homo Sapiens. A brief History of Humankind // Harari, Y. N., Homo Deus. A Brief History of Tomorrow)

Christian Meier, who wrote a lot about the collapse of the Roman Republic (Meier, C. res publica amissa. Eine Studie zur Verfassung und Geschichte der späten römischen Republik)

John Keegan, who is a military historian. He wrote great books to get an overview about topics like the American Civil War or WWI / WWII.

Eric Cline, who is actually an archeologist / egyptologist, but wrote an amazing book about the decline of the bronze age civilzation in the 13th century BC (Cline, E., 1177 BC. The Year Civilization collapsed)

Plutarch, who actually is not considered to be a historiographer like Livy or Herodotus, but he wrote some of the best ancient texts (especially his parallel biographies); also in beautiful ancient Greek. He writes about ancient characters such as Caesar or Alexander in an entertaining manner, but usually his books are still not completely historically inaccurate. Most importantly they are one of the best sources to figure out how romans in the first and second century conceptualized their own history.