Both of these, but probably more WWII due to the sheer devastation and loss of life that it resulted in throughout a large part of the world. In contrast, the Cold War was full of tension but relatively short on actual fighting with the exception of some proxy wars that were nowhere near as bloody as WWII was.
In the grand scheme of things, though, I do think that the 20th century will be heavily remembered as a time when concepts such as national self-determination, racial equality, freedom, liberalism, et cetera ultimately triumphed throughout much of the world.