Given that the last line contains the only metaphor in the poem -- all of the other comparisons that came before are similes -- and because metaphor is generally believed to be the strongest method by which a comparison can be drawn, the last line of the poem seems to be the most truthful, communicating the inevitable eventuality of a certain deferred dream.
Because a metaphor says that one thing is another, instead of saying that it is only like another, a metaphor is more forceful than a simile. The narrator suggests several possible outcomes for a dream deferred (and it seems to be the specific dream of racial equality, based on the title of the poem), all via simile, before he reaches the final metaphor. The final line compares the deferred dream to a bomb by describing it as something that would be able to "explode." The implication, here, is that if the dream of racial equality continues to be denied to African Americans, then the result with be both violent and dramatic, and it will impact everyone, not just the African-American community.
Further, because this line is italicized, and we typically italicize words that we wish to emphasize, it seems that this comparison is underscored as the truth in three ways: it is the only metaphor in the poem, it is visually different from the others by being the only line that is italicized, and it is the only line in the answer to the initial question that stands by itself. For these reasons, I believe that the final line of the poem, then, is the inevitable outcome of the deferred dream.