Bringing a civ back from the dead by liberating them - from then on, they become near-permanent allies in gratitude.
Optimism was a key focus in V, but it's the opening to Brave New World that takes the cake, talking about humanity harnessing technology and arts to improve themselves, and of us putting aside differences to overcome challenges like wars and ambitious evil men. It's these kind of touches that brings tears to the anthrophiles among us.
The opening cutscene of 6 has a father and daughter discussing making their mark in history. While the father is depicted as doing great feats throughout history, one wonders what his daughter is doing. At the conclusion, it's revealed the daughter is going on a manned mission to Mars - and has a picture of her father on the pilot dashboard.
The theme for Greece in IV is derived from the oldest known complete musical melody, believed to have been written in memory of a woman named Euterpe, telling people to cherish their time and live to the fullest. Even thousands of years after Euterpe's passing from the world, she's still in a way remembered and honoured.