T.S. Eliot’s “The Waste Land”, of course, is one because it changed the course of modern poetry and dealt with the greatly-changed Western world after WWI. It embodied the angst and ambiguity of the early 20th century.
Allen Ginsberg’s “Kaddish” would be the other. I revisited it last night on Ginsberg’s 63-minute live reading on the rare old Rhino Records boxset Holy Soul Jelly Roll. It’s an epic poem about madness, death, and how a very insane mother in a family affected everyone in the family and destroyed it as well as marking her young son Allen for life, putting him through hell (Ginsberg quotes Conrad’s “The horror! The horror!”) no child should ever experience and simultaneously making him into the empathic great poet of the Beat Generation. The reading is fantastic; no other 20th century reading or poem comes close to matching the depths of anguish in this epic.
(I discuss “Howl”, in the Nov.2. 2019 entry, as one of the three greatest twentieth-century poems.)