Dean

Reasons: 1. WW2 saw the largest military buildup the world has ever seen, with more men, tanks artillery pieces, ships, aircraft, and supplies produced and fielded than any other conflict.
2. The conflict and it's end shaped world history right into the present time.
3. It resulted in more casualties, both military and civilian than any other war in human history.
4. The colossal amounts of money spent dwarfed anything that came before and the military industrial complex that resulted from the war ended up shaping the world economy for decades afterwards.

I could go on, but these reasons alone should suffice.

Dean.