Gotta go with WW2. Ever since I was a kid, flipping through the Time Life series on the Pacific Theater, that period has fascinated me.
Close runner-up would be Age of Sail, simply because it never ceases to amaze me that men could build ships like that, sail them without sinking or foundering, navigate them without getting lost, and use them to (for the period) fight effectively against other ships, and all of this even before the Industrial Age, let alone the Information Age.