It is difficult to pinpoint one war and say it is the most "important" war in human history.  There have been many conflicts which have greatly altered the ripples of history, sometimes greatly affecting the world for hundreds or thousands of years (Greco-Persian War, the Punic Wars, the Battle of Kosovo, the Manchu Wars in China, the Napoleonic Wars, the United States Revolutionary War, the Russian Civil War, etc etc etc).  For the sake of argument, I will give my examples of one of the major conflicts which abruptly turned the world on its heel and altered history indefinitely.  I feel, that based on the historical context, these conflicts were the "most important" wars/battles.

The Roman defeat at Teutoberg Forest (http://en.wikipedia.org/wiki/Battle_of_the_Teutoburg_Forest)

I feel that this had the greatest impact on Western civilization.  By not only losing, but losing embarassingly, at the ambush at Teutoberg, Publius Quinctilius Varus signed the death note to the Roman Empire.  It's all a very slippery slope, but I feel that it is a very safe argument to make.  He doomed the Roman Empire and greatly altered history.

By being lured and trapped into the fight, Varus and his legions were completely surrounded and vulnerable to a much smaller, clumsier, and less armed German army of brigands.  The three legions were the proverbial "sitting ducks" and were slaughtered in due form.  This defeat was overwhelming, and not even based on numbers alone, with the Romans losing between 15 and 20 thousand soldiers. 

They key to this defeat is morale.  The defeat gave the Germans a newfound sense of pride.  They had defeated, handily, the great and powerful forces of civilization!  Hermann, the German leader (who had beguiled Varus into the trap, as Hermann [Arminius] was a former Roman hostage who had become Varus' advisor) was exhonerated briefly (assassinated) as a king, and I feel is the inspiration for Seigfreid, the heroic dragonslayer from 1st century Gothic  epics.  Through this character and oral history, Hermann lived on to inspire German nationalism and defiance to the Roman Empire.  The success of this battle led to a resurgence of German resistance to Roman occupation, and the eventual abandonment by Rome of conquering Germany.  This is the infamous battle, was said to have driven Augustus Caesar partially mad, and he could be heard striking his head against doors and walls while mumbling or screaming "Varus!  Give me back my legions!"  Pretty heavy stuff if you consider just how powerful, sophisticated, and intelligent Augustus Caesar was.

In the short term, this had minor effects.  The Roman Empire, under Augustus' heirs, tried vainly to fulfill the dream of conquering and civilizing Germany.  Long story short- armies were amassed under various "Caesars", battles were fought, but mutinies and low morale led to an eventual stalemate on the issue, drawing the border between The Roman Empire and Germany at the Rhine.  The Roman Empire set its sights on other areas, as it still had problems with Parthians, Illyrians, Judaea, and (very important to note) an upstart, problemic Jewish carpenter name Jesus who was starting quite the uproar in Israel.