Yeats' "The Second Coming" is the most referenced poem of the 20th century; "He Wishes for the Cloths of Heaven" might be the most beautiful;
Pound's "Hugh Selwyn Mauberly" is dense enough to stand up to dozens of readings but accessible enough not to require poring over to get a drip of info out of.
Ted Hughes' "Daffodils" is wonderful
Wallace Stevens: The Snowman
Stanley Kunitz: The Wellfleet Whale
Randall Jarrell: The Death of the Ball Turret Gunner
Frost: Nothing Gold can Stay
Wilfred Owen: Dulce et Decorum est