Who are the most important poets of World War I?

A number of influential English poets sprang from the years of the Great War. Unfortunately, many of their voices had been silenced by death before the war's end. Notable British poets who offered their perspectives on the war and/or influenced later poets include Rupert Brooke, Siegfried Sassoon, Wilfred Owen, and T. E. Hulme.
Of these, Rupert Brooke was the one whose poetry reflected the nationalism and patriotism that characterized the young British men at the beginning of the war. Winston Churchill honored him by saying,

He was all that one would wish England's noblest sons to be in days when no sacrifice but the most precious is acceptable, and the most precious is that which is most freely proffered.

His famous sonnet "The Soldier" is representative of his work. It begins with the memorable and heart-wrenching sentiment: "If I should die, think only this of me: That there's some corner of a foreign field that is forever England." He died on a troopship in 1915.
Siegfried Sassoon is famous for introducing realism into war poetry. His sarcastic and biting verse called out the church, the military, and politicians for romanticizing and drawing out a war that was murdering young Englishmen. In "They," he derides a "Bishop" for describing how the soldiers "will not be the same; for they'll have fought in a just cause." Ironically, he twists those words and points out that their bodies—blinded, wounded, with amputated limbs—will certainly never be the same. Sassoon survived the conflict.
Wilfred Owen met Sassoon when Owen was hospitalized for shell shock (now known as post-traumatic stress disorder) after fighting a few months in the Battle of the Somme. After Sassoon's influence, Owen wrote realistic war poems such as "Dulce et Decorum Est" and "Disabled" that portrayed the war in moving language. He died in action a week before the war ended.
Another poet who wrote around the same time period and fought in the war is famous not for his war poems but for his influence on twentieth-century poetry. T. E. Hulme was one of the founders of imagism, a poetic movement that valued brevity and clear imagery. He died in battle in 1917.