My vote is for Serah's swimsuit for Final Fantasy XIII-2.
What?
Oh fine, I'll be serious. I will agree with everyone who has said Lair of the Shadow Broker. Playing through that was the first time I actually felt like games had become more than 'just games', and were actually interactive entertainment worthy of a respect and popularity that can be placed alongside cinema, literature and, uh, whatever else might take your fancy. If for no other reason than the performances.
Borderlands I will agree with, some of the Dragon Age stuff (I quite liked Darkspawn Chronicles, it was something fun and different, and better than lame zombie DLC), and Kingdoms of Amalur offered a meaty DLC. As long as its rich in content and takes longer to get through than it takes to load, I'm happy with it.