Conquering a major nation as a minor nation. Bonus points if you're playing a generic nation and are nothing more than expansion fodder for the major.
Defeating The Washington Brotherhood as the Yakama Nation or Olympus Tribe is a good example
A very clear moment of awesome relating to this is defeating Old Country, The Washington Brotherhood, The Bone Dancers, and/or the Troll Warren as the Timberline. Imagine being the biggest bads of the wasteland, and as you start stomping around, seeking to claim a seemingly defenseless territory, you get the resistance of a lifetime, that ends once you don't exist anymore.
As of an update, Lanius receives the option to turn against Caesar and take the Legion for himself. In doing so, he forms the Res Publica, a new Roman Republic. Lanius, "The Monster of the East" is one of Caesar's top generals and can establish a government with a good chance of lasting after his death, something that not even Caesar could do. Of course, he does have a little bit of help from the Cult of Diana.
. Bonus points to the Odious King and Tlalocan.