Which battle do you think
was most important in turning the war in favor of the
Allies? Why?

It was without a doubt battle of Stalingrad.

Explanation:

During WW2 there were many important battles, but this is certainly the turning point.Germany wanted to crush the Soviet Union and thus move their troops to other fronts.Still, the battles lasted for months, hundred of thousands soldiers died but the Germans didn't managed to capture the whole city.After this event the tide of war has totally changed.