I know that's a biiiig question. Interpret "important" in any way you like. And the pieces of writing can be just that--whether they're works of Literature (with a capital L) or a warning label.
For me, those pieces of writing would include three of Shakespeare's plays: The Tempest, Othello and Macbeth. They would also include T.S. Eliot's The Love Song of J.Alfred Prufrock, Victor Hugo's Les Miserables and Sappho's Odes. And, I must not forget NSC-68, Christine Jorgensen's autobiography (which I read as a teenager in the local public library) and The City of Ladies.
Oh, here's another: Everybody's Bike Book by the late, great Tom Cuthbertson. Mechanics tend to think spatially and visually rather than verbally, so anyone who can turn bike mechanics--or any kind of mechanics--into prose that's understandable, much less enjoyable to read, is a truly special kind of writer.
Here is something I am sure Tom, rest his soul, would appreciate: