World War One
World War Two
Fighting purely in trenches is absolute hell.
WW1 caused WW2, created the Soviet Union, and put into motion many other things which caused virtually every war since
World War One was probably worse and there is one reason, the world had never seen such a bad war, most countries were thrown into the war before they knew it, and it also helped the world see that the time for peace and to end war had to come soon. The aftermath sucked too, if the world had just helped Germany get back on it's feet, the Nazi party wouldn't have taken over, and World War 2 would never happen.
If you want to be realistic, the first world war worked on the tension of all nations that later started ww2.
World War 2 resulted in more loss of life, the US became the unofficial Police Force of the World, innocents were gassed to death in rooms, and the Communists got momentum because of the Fascist opposition. The Jews got Israel, and have turned into bad people since. Not to mention the US nuking innocent people in Hiroshima and Nagasaki, when we likely could have run an extensive carpet bombing and achieved the same results. Every current world problem spawned off of World War 2 - Iran/Iraq War (oil/world police), Syria (communist/capitalist proxy war), nuclear weapons are still rampant (but not deployed, Israel v. Gaza, 9/11 (terrorism for going into Middle East), ISIS (backlash against US policing), the UN is worthless (US world police), and there is a current conservative backlash worldwide. In comparison, WW1 had less innocents die; the majority of those who died were in the military, and at this time if WW1 wasn't a thing, they would have been sent to fight in trenches elsewhere. Trench warfare was horrible, but obviously not as bad as nuclear weapons and the blanket of radiation that destroys the land and water supply (Chernobyl, anyone?). And quite frankly, if the argument for WW1 being worse than WW2 is the fact that it caused "all" future wars, my response is this: have you read the Bible/The Quran/The Torah? If these monotheistic religions that all say that "they are chosen/all other are enemies of God" didn't exist or at least weren't as prominent, almost ALL wars could have been prevented, except for those that were over territory and resources only. The only thing I can say that could make WW1 close to WW2 is that the men who died on the battlefield died for nothing. Everything went back to the way it was, except Germany became economically unstable and resulted in the rise of Hitler.
World War II had a lot more death, and plus with the invention of the machine gun, tank, and atom bomb, people had more fear of being attacked in their homes. Also, Hitler killed thousands of Jews, and still tortured many others.
We shall go on to the end. We shall fight in France We shall fight over the seas and oceans. We shall fight with growing confidence and growing strength in the air. We shall defend our island whatever the cost may be We shall fight on beaches, we shall fight on the landing grounds, We shall fight in the fields and in the streets, We shall fight on the hills. We shall never surrender.