Several different stanzas of "The Love Song of J. Alfred Prufrock" could be considered important depending on the interpretation of the individual reader. However, the following stanza is arguably the most significant for thematic reasons:
For I have known them all already, known them all:
Have known the evenings, mornings, afternoons,
I have measured out my life with coffee spoons;
I know the voices dying with a dying fall
Beneath the music from a farther room.
So how should I presume?
This stanza thoroughly lays out Prufrock's existential state. He has lived a monotonous, empty life that he does not find fulfilling. The image of measuring out life with coffee spoons is meant to emphasize how trivial Prufrock's life is. The emphasis on the different times of the day also shows how Prufrock's life never changes, with each hour going by with little to inspire or wake Prufrock from his malaise. The section about Prufrock hearing voices dying with a dying fall is his way of acknowledging his mortality. The text suggests that Prufrock is a middle-aged man, and he knows ahead of his monotonous life lies only old age and death.
These qualities are what makes the poem so poignant: Prufrock is a frustrated and ordinary man desperate for meaning, and he puts these frustrations into everyday terms that make the existential emptiness more real and painful.