I would have to say George Orwell. Ranked by the British daily The Times as second on its list of "The 50 greatest British writers since 1945", Orwell may not be the best novelist or poet, but he is no doubt, one of the most intellectually honest writers of all time, which makes him the greatest in my book.

As the author of Animal Farm (1945) and 1984 (1949), Orwell wrote about topics that are as relevant and controversial today, as they were some 70 years ago. Tackling themes like political propaganda, state surveillance, and the dangers of totalitarian rule, he is a writer ahead of his time. Who would have thought that his Orwellian society where citizens are subjected to omnipresent surveillance and invasions of privacy, and where governments make use of "alternative facts" and propaganda to control its citizens would become everyday realities.

However, what really makes these writings stand out is that they were straightforward attacks on what he viewed as intellectual hypocrisy. Literary critics described him as possessing an "uncompromising intellectual honesty made him appear almost inhuman at times". As a writer, he was therefore uncompromising, bent on telling the truth in a "straightforward common-sense way". 

Orwell's influence in language and writing is also undeniable. As early as 1946, in his essay "Politics and the English Language", he wrote six writing rules that are still taught and quoted today. His six rules were: 

"(1) Never use a metaphor, simile or other figure of speech which you are used to seeing in print. (2) Never use a long word where a short one will do. (3) If it is possible to cut a word out, always cut it out. (4) Never use the passive where you can use the active. (5) Never use a foreign phrase, a scientific word or a jargon word if you can think of an everyday English equivalent. (6) Break any of these rules sooner than say anything outright barbarous."

And thus, because of his enduring relevance and his intellectual honesty, I would consider Orwell as one of the greatest writers of the 20th Century.

Source:

George Orwell. https://en.wikipedia.org/wiki/George_Orwell