Retail expansion packs are a viable means for game developers and publishers to increase the longevity and improve the core qualities of their best game designs. The best of these take an already great game and manage to make it even better than before by introducing fundamentally new design elements that thoroughly improve upon the original game. Many great games receive suitably great expansion packs, but here is the greatest expansion of all this year:
Command & Conquer Yuri's Revenge

Publisher: EA Games
Developer: Westwood Studios
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Most expansions simply give you more content or more characters to play. However, Command & Conquer Yuri's Revenge, the expansion to last year's highly enjoyable real-time strategy game Red Alert 2, was a cut above the rest. Yuri's Revenge features 14 new campaign missions for the returning Allied and Soviet factions. You can also play as the all-new Yuri faction in skirmish and multiplayer battles. Yuri has unique units that are straight from a sci-fi novel--flying saucers, mutated brutes, psychic dominators, and more. Perhaps the greatest aspect of the expansion is that the new units for the Allies and Soviets completely change the tactics for these factions too. It's a great example of how an expansion pack can completely overhaul an existing game.
Other Nominees:

Baldur's Gate II: Throne of Bhaal
Diablo II: Lord of Destruction
EverQuest: The Shadows of Luclin
The Sims: Hot Date