Undoubtably the 40s for WWII. The real question should have been the second most interesting decade, which in my opinion is probably the 2000s as that's when everything went wrong.
We'd won the Cold War in the early 90s, the economy across the whole west was looking up, the US was about to start reaping the benefits of the tech boom, the UK had broken stagnation, militant unionism and balanced its budget. We should have settled down to a nice boring period of paying down debts and nudging the Chinese to buck up their human rights, maybe venturing out into short, popular and successful humanitarian wars ever so often. For a while we did .....then it all went a bit wrong with war, more war and toxic debt.