Oh, I'd say Enemy Ace. An interesting look from the enemy's point of view. Something never tried before, at least as far as a main character was concerned.
I've always wondered if the original intention was to show a WW II German pilot, but perhaps the wounds of the war were still a little too fresh? It was almost another decade before Blitzkrieg came along and even that didn't last very long.
The Unknown Soldier is a great character with a lot of great, under appreciated stories, but he is essentially a secret agent, a 007 of his era. As intriguing a character as he is, I think the other side of the coin aspect of Enemy Ace make him the significant character.