Red Alert 2: Yuri's Revenge is a shining example of what an expansion can be. A lot of refined mechanics, better fleshed out factions (plus one new one!) and three new campaigns with just as many FMV cutscenes as the previous iterarion.

Honorable mention and this kinda doesn't count but Mental Omega is a mod for Yuri's Revenge that has between 3 to 4 times more content than the base game. There's a dozen subfactions split between four subfactions and inregrates features from later C&C games while introducing a lot of it's own stuff. It has some "borrowed" voice lines from other games but there are some units that have original voices which is pretty cool. No FMV video though. It's suprisingly deep for what it was based off of and it has become one of my favorite RTS experiences.

Side note:

This is such an unpopular opinion of mine but while Dead Money is cool my favorite of the FONV expansions was Old World Blues because of the goofiness of the characters contrasted with the manmade horrors of the big empty. I found Lonesome Road to be meh and Honest Hearts to be uncomfortable with how it seems to embrace the concept of the "White Man's Burden" uncritically.