What is the most important era in history of English literature?

Another thing to consider when trying to answer this question is how one defines English literature. For centuries, English literature was defined as writing created by white men. When greater numbers of women writers started producing notable and critically-acclaimed work (including icons such as Mary Shelley, Christina Rosetti, Jane Austen, and Virginia Woolf), English literature's scope widened.
In more recent years, more ethnic and racial diversity has been seen in English literature. This recent diversification in English literature showcases more gender and ethnic difference. This means that these authors' viewpoints will continue to expand and shift the long-time perspectives of homogeneous writers of white European extraction.
The Booker Prize, considered England's top literary prize, has in recent years been awarded to increasing numbers of female and non-white English novel writers (like A. S. Byatt and Kazuo Ishiguro), marking a change in that prize's usual winners. The prize is still generally seen as tending to award more of its top prizes to young white male novelists, and this is a frequent criticism that suggests this issue is still prevalent. However, in 2017 there was a larger than usual number of women of color in the "long list" of writers being considered for the award.
Going forward, increased diversity in English literature will continue to be an important aspect of how literature reflects cultural evolution, human rights, regime changes, and shifting populations, among other things.
https://www.elle.com/uk/life-and-culture/culture/news/a37347/man-booker-prize-2017-longlist-diversity/

https://www.thegryphon.co.uk/2017/10/23/man-booker-prize-2017-a-disappointing-lack-of-diversity/

There is no era in the history of English literature that can, on any objective grounds, be considered "most important," as different aspects of English literature developed in different periods. At best, we can only discuss which era was most important for some particular developments.
The late medieval period is important because is was one in which "English literature" per se evolved. It was in this period that literature in the vernacular began to flourish and the English language as we now know it evolved. It marked the beginning of accentual-syllabic prosody which dominated English poetry for many subsequent centuries. 
The early modern period was extremely important for the development of English drama and poetry, with Shakespeare and Milton being among the towering figures of the period. 
The eighteenth century was most important for the development of the English novel as a genre and also for the birth and growth of periodicals. It was a major period of growth for literacy and also for book production.
The nineteenth century was a period of a great proliferation of literary works, in part enabled by an explosive growth of mass literacy and technologies which enabled inexpensive mass production of printed works.