Machado de Assis's most famous poem is usually taken to be the sonnet "To Carolina" ("A Carolina"), although there is no official ranking. Wikipedia describes it as the best piece in the author's poetic work.
Machado wrote it for his wife, Carolina Augusta Xavier de Novais, whom he married in 1869 and who died in October 1904. The sonnet opens the book Relics of an Old House (1906) and begins with the line "Querida, ao pé do leito derradeiro" ("Darling, at the foot of the last bed"), addressed to her. The poet Manuel Bandeira considered it one of the most moving pieces in Brazilian literature.
Another much-read and much-discussed poem is "Vicious Circle", in which the firefly, the star, the moon, and the sun personify envy and jealousy. According to the commented reading, it portrays the dissatisfaction native to the human being, and it was included in the Poesias Completas (1901).
Machado's poetry gathers four books: Crisálidas (1864), Falenas (1870), Americanas (1875), and Ocidentais (1901).