The Cold War would be my answer, but technically there was no direct combat between the two superpowers. I really find World War II interesting, namely because of Hitler's regime, Italy being a fascist state and also the Japanese attack on Pearl Harbor. I also find Poland's role in the war very interesting too - it's amazing how much they endured.
But I think my answer would be Vietnam. It had a real nightmarish feel to it, primarily because of the environmental surroundings which the soldiers were forced to fight in. Some really interesting strategic elements, too.
How about you?