The First World War; 28 July 1914 – 11 November 1918.
The Second World War; 1 September 1939 – 2 September 1945.
A FEW reasons. One, I know more about WWII, and I want to know more about WWI. two, WWI directly caused WWII, so understanding why WWI happened and what happened at the end will help make sense of WWII. Finally, WWI was the first modern war in weaponry - submarines, airplanes, huge artillery, chemicals, trenches, machine guns, et cetera. It's interesting to see how the generals of WWI stumbled around with the weaponry.
This is the more fertile ground because it is underreported thus I find it more interesting and more enjoyable.
I am very interested in Josef Mengele and all the possible experimentations that happened during that time. It makes the WWII my favorite subject when it comes to war (and I am not only speaking about what happened in Auschwitz and such).