Each war has such a heady blend of death, destruction and hate it is just so hard to pick a favorite . That said, WWII had a much bigger scope and involved way more air and sea technology. Plus my Dad was in the last big one (or, “the last one we won” as he says.)
I’m a history buff and have even written some books on it. For some reason, I am not all that interested in WWII, although the older I get, the more interesting I find it. I picked WWI, but I’m not overly interested in that, either. Now the American Revolution–definitely. Civil War? Somewhat. For me, the older the war, the more interesting the warfare.
I agree with JohnT, WWI. Possibly the stupidest war that, in no small way, caused WWII.
WW1 simply because WW2 is fairly cut and dried as to who the major villains were (if you ignore Stalin’s atrocities in light of the hammering the Nazis dealt to the USSR that is).
In WW1 everyone is a dick. Germany is a dick. France is a dick. England is a dick. Russia is a dick. The USA is a dick. Italy is a dick. Ottaman is a dick. Autria-Hungary is a dick. Even a bunch of little European countries are dicks; dicks way out of proportion to their size and strength (Romania, i.e). Heck, as far as I can tell the only nation that wasn’t a dick was China
The consequences of the war, and the over-promising of the politicians about carving up Germany in the peace has lead us to consequences that are still with us today: Iraq, for example.
A good read is Paris 1919 for just how screwed up the peace process was.