Like the title! I normally am very critical of Expansions, but sometimes they build upon the base game in very unique ways. I love Dead Money for Fallout: New Vegas, it has the sharpest character writing and turns the game into a Survival Horror experience while remaining authentically "Fallout."
What are y'all's thoughts?
Mists of Pandaria for WoW, not only really great atmosphere but also introduced many crucial mechanics that improved the game immensely
Tours and Tournaments for CK3, plugged the peace boredom gap in game
Every major expansion for Civilization 4-6
Stellaris: Utopia, Megacorp (though more of a free patch that came with its release then the expansion itself), Nemesis, Machine Age
Warhammer TW2: Tomb Kings, waited for that faction since beginning of series and didn't get disappointed