Normally, it's hard to know how important something is to history when history is still happening around you. (Unless that thing is something deeply shocking, like the bombing of Pearl Harbor or 9/11.)

Among other civil rights leaders, "I Have a Dream" originally had a mixed reception. One organizer from Mississippi remarked, "We never had time to sleep, much less dream." This remark captures the general criticism that Dr. King was too idealistic, too preachy, and too removed from the action on the ground. (Source)

Back in the 1960s, Americans were generally divided on MLK. According to some public opinion studies, his detractors may have even outnumbered his supporters. (Source)

But not everyone was a critic. After "I Have a Dream" capped off the March on Washington, President John F. Kennedy apparently said, "That guy is good." (Source)

Way to make an understatement, JFK.

"I Have a Dream" has grown in reputation after Martin Luther King, Jr.'s death. Sort of like James Dean, Kurt Cobain, 2Pac, and other celebrities who died too soon, Dr. King's premature death enlarged his image in the eyes of the public.

As a result of his Nobel Prize, his increased reputation over time, and the successes of the Civil Rights Movement, "I Have a Dream" is now known by everyone and their grandmother. Scholars consider "I Have a Dream" to be the speech of the century. (Pro-tip: anytime something comes before the phrase "of the century," there's a good chance it will be the main event on a history test.) (Source)

See for yourself. A quick Google search of "I Have A Dream" turns up millions of results. In terms of the anecdotal evidence—the sheer number of people, students, teachers, foreigners, and little kids who have heard parts of this speech—"I Have a Dream" is unquestionably on the Mount Rushmore of oratory.