The "most important" Shakespeare play has changed over the centuries.
In the 1800's, Macbeth was seen as the seminal work being portrayed by William Charles Macready, Edwin Forrest, Samuel Phelps, Charles Kean, Henry Irwing, and the famous Edwin Booth.
With the advent of psychoanalysis in the 20th Century, our fascination shifted from the Scottish Play, to Hamlet. Indeed, the famous scene in Gertrude's closet never included a bed until Sigmund Freud's invention of the Oedipus Complex.
Another medium that strongly influenced our perceptions of this play was the invention of the motion picture. Indeed when John Gielgud was asked to play the title role in the first major film version of the play, he turned it down because he did not trust the medium; he didn't think it would last. The role went instead to Lawrence Olivier.
However, as we head deeper into the 20st Century and turned the corner into the 21st Century, a new play seems to be looming on the horizon. With people living longer than ever before and a drastic generation gap created by continually advancing technology, King Lear has been gaining importance and popularity. The play seems to be asking the questions that we are facing in our world as we try to redefine the place of our elderly parents and leaders who grew up in a world far different than our own.
As we see our most respect Shakespearean actors age (Ian McKellan, Patrick Stewart, Christopher Plummer, Derek Jacboi, Ben Kingsley, etc.), I have a feeling that we will be seeing many "seminal" productions of this play over the next decade or so.