William Shakespeare is widely acknowledged as the most important figure in English literature. Although some people consider him the "best" author in the English language, that point of view is probably overly simplistic. There is no real way to measure objectively what writer is "best". Instead, importance really is not a quality judgement but one about how a writer becomes foundational in a literary tradition.
Roland Barthes famously stated that "literature is what is taught, period". In other words, while we read or watch and enjoy many imaginative works, the foundational ones to our culture are defined by what works we invest in handing down within our educational curriculum and thus what works become a ground of shared cultural knowledge. Thus Shakespeare for English literature, as Homer for the Greeks or Virgil for the Romans, has become the figure central to the literary tradition of our language.
The first reason for this is that he wrote at the beginning of modern English. Chaucer, who might have equal claims based on subject matter and literary innovation wrote in Middle English. Most English schools were focused on Greek and Latin classics through the nineteenth century. When writing in the English vernacular was introduced into school curricula, first in the Dissenting Academies, and later diffused through a wider range of instruction, Middle English had become alien to ordinary speech. Although Shakespeare was appreciated by some earlier writers, his reputation began to grow as English literature was gradually introduced in the curriculum. He was a particular favorite of the Romantic writers and accounts of his "unique genius" were often used by the Romantics to justify their own freer literary forms against the more strict formal devices of the Augustans.
Shakespeare's plays were crucial in the development of English drama, and are especially prized for the sheer range of human experiences they reflect, including both comedy and tragedy and characters from all stations in life from "rustics" to to kings. They include both imaginative and historical works. His use of language is widely admired, with many of his phrases having become so much part of the English language that they are used by people who may not even be acquainted with his works. His characters also, such as his doomed lovers, Romeo and Juliet, or his quarreling lovers, Beatrice and Benedict, have become patterns for innumerable subsequent literary works.