What could be considered the most important battles of World War I and II?

It is important to remember that these were both massive
conflicts, with hundreds of battles on multiple continents, so "most important" is going
to be a matter of opinion.  But in general, historians often agree on a few that were
critical turning points in both wars.  Here are a few that might
qualify:

World War
I:

Battle of the Somme - This
massive battle in 1916 was important in that it demonstrated what a long, bloody and
futile war it would be.  The British launched this offensive and in the first day lost
60,000 men.  600,000 would die before the battle was
over. 

Chateau Thierry - This
battle in 1918 near the end of the war represented the "last gasp" of the German forces.
Once the attack was stopped, mostly by US troops, the Kaiser had to sue for peace as his
economy and army began to collapse.

World War
II:

Battle of Stalingrad - The
turning point on the Eastern Front, the entire German 6th Army surrendered, and from
that point on, Hitler's forces in the Soviet Union were in almost constant retreat. 
Germany never had much of a chance to win in Russia, but Stalingrad marked the beginning
of their doom.

Battle of Guadalcanal -
This drawn out, six month battle in the Pacific pitted Japanese and
American troops that were both, at one time or another, cut off from supply.  Fought at
about the same time as Stalingrad, this is often considered the turning point of the the
war against Imperial Japan.