Yesterday, I purchased the Citadel DLC for Mass Effect 3. While the add-ons that followed ME2's Lair of the Shadow Broker were solid, none of them measured up to that one in terms of character interaction, memorable moments and locations, and overall fun. Until now.
Citadel is an amazing sendoff to the Shepard story that simultaneously honors and lampoons the series. The story is intriguing, the amount of content is rich (especially for being $15), and the character interactions are some of the trilogy's most memorable moments. This might just be the most fun that I've had with any DLC period, which is surprising, considering that Leviathan and Omega were slightly underwhelming.