World War 1
World War 2
Well it would make sense that the first is the best choice cant have a part 2 without a part 1
WW1 undoubtedly led to WW2. WW1 taught us that you should not tangle alliances, and should not punish countries as Germany was punished. They come back with vengeance.
Never humiliate a country.
More destruction. Therefore we have more to learn from.
We know more things in World War 2
Letting genocide and the thought of obtaining vast tracts of land for Germans was a winning proposition. Also, From both the Nazi's and the Allies great questions of morality were considered about war crimes, war criminal acts, who is responsible when an order is given, and the like.
Not to centralize command structures in tactical positions to off field personnel.