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?
The Rise of the Witch King expansion for Lord of the Rings: The Battle for Middle Earth 2
If you like RTS games and/or Lord of the Rings these games are pretty easy to pirate and there is an active modding community.