Mass Effect 2
Great progression of a story thread that ties in all the way back to early ME1. Interesting character developments. Adds a few distinct and cool looking atmsopheric environments.
For the detail oriented, the Shadowbroker dosier and video clips have some great background info on characters throughout the games.
My personal favorites for the dosiers; Tali's ecalating off again, on again installation (lol, frustrated much?), a certain favored video for some of the crew, and a misguided extranet search for an apt but non-existant dinosaur/alien combo.
For the videos clips, a one sided boxing confrontation between two old advesaries (when you see it, I dare you to tell yourself you didn't feel a flicker of satisfaction), and an assassination and a judgement.