I start with Martin Luther. He was not the first one who translated the Bible into German, but his translation was most influential.
When Luther died almost every second German household a German Bible. Before him, almost no one (a few highly educated academics had Latin Bibles). So, his translation contributed to the development of a written German language that was understood and accepted in the whole area where a German dialect was spoken. It was a matter of luck that Luther lived in a rather central part of Germany, so that his language was accepted in the North and in the South, in the West and in the East.
Also, Luther invented a number of new words, like Beruf, Lückenbüßer, friedfertig, wetterwendisch, Machtwort, Feuereifer, Langmut, Lästermaul, Morgenland, and he coined a lot of expressions, like "sein Licht unter den Scheffel stellen", "sein Scherflein beitragen", "der Mensch lebt nicht vom Brot allein", "Stein des Anstoßes sein", "mit Blindheit geschlagen sein".
EDIT:
Michael Staengl translated “Beruf” with “paid job”. This is inappropriate. The literal translation of “Beruf” would be “vocation”. In fact, Luther meant that God gave you a certain talent and that he called you to work according to this talent. “Beruf” means that you learn for years to do your work and that you keep working in this field lifelong. This is the reason why in German virtually every job is a “Beruf” which requires a three-year apprenticeship or even longer training. A typical conversation in German could be like this:
A: “What is your job?”
B: “I am currently selling homes, but my Beruf is being a teacher.”
A job is something what you may do just to earn money and you may do it temporarily. And you may change jobs during the course of life. A Beruf is something that you have learned to do and that you may do lifelong, giving you an identity. In the English language “profession” comes close to “Beruf”; Beruf, however, is not restricted to academics. Beruf is also blacksmith, shoemaker, hairdresser, cook, nurse, accountant - whatever.
So, Luther not only coined a new word - he brought a theological concept into society and economy. The sociologist Max Weber wrote an intriguing essay on this topic:
END OF EDIT.
So, Luther was doubtless the most influential German author.
Another influential author was Johann Wolfgang Goethe. Goethe used an elaborate, somewhat sententious language. But he also coined a lot of expressions which became part of the common language, like “Wer nie sein Brot…”, or “Mein schönes Fräulein, darf ichs wagen, Mein Arm und Geleit ihr anzutragen? Bin weder Fräulein weder schön, Kann ohngeleit nach Hause gehn” or “Dass ich erkenne, was die Welt Im Innersten zusammenhält”.
Then, the Grimm brothers. Jacob Grimm and Wilhelm Grimm were linguists. Their collection of fairy tales, however, was read by practically every literate German. Children heard the stories which were read by their parents or nannies. And each German, until today, knows the key sentences from these fairy tales. And expressions like “Königskinder” have become part of the common language.
Hugely influential was also Wilhelm Busch, who is also known as the inventor of comic strips. He wrote funny stories, often in rhymes, and used comical expressions. Many of his verses became also part of the common language, e.g. “Rotwein ist für alte Knaben eine von den besten Gaben”, “Und dieses war der erste Streich, doch der zweite folgt sogleich”, “Musik wird oft nicht schön gefunden, Weil sie stets mit Geräusch verbunden” and many more.
I would also mention Heinrich Heine. He was a romantic poet and he also was a political satirist. He also coined a number of expressions that everyone knows “Ich weiß nicht, was soll es bedeuten…”, “Das ist schön bei den Deutschen: Keiner ist so verrückt, dass er nicht einen noch Verrückteren fände, der ihn versteht”, “Denk ich an Deutschland in der Nacht, Dann bin ich um den Schlaf gebracht”, “„Dort wo man Bücher verbrennt, verbrennt man auch am Ende Menschen“, “So ein bisschen Bildung ziert den ganzen Menschen”.
