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?
Blizzard during their golden age created nothing but bangers: Starcraft Brood War, Warcraft 3: The Frozen Throne, and Diablo 2: Lord of Destruction. They turned great games into some of the best their entire genre has to offer.
I also have a soft spot for World of Warcraft: Burning Crusade. I started WoW a couple of months after release, but I got really into it with Burning Crusade.
Additionally, Dawn of War 1 and 2 had some really fun and good expansions. Soulstorm's campaign was incredibly fun.