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 husband and wife discussing making their mark in history. While the husband is depicted as doing great feats throughout history, one wonders what his wife is doing. At the conclusion, it's revealed the wife is going on a manned mission to the Moon - and has a picture of her husband on the pilot dashboard.