If you have played Grand Theft Auto 4, but not the two DLCs released for it (The Lost and the Damned ..+ ..The Ballad of Gay Tony) you are missing out.
Both have won major awards and builds onto the original GTA4 storyline... also, they tie to various parts of GTA5 as well
For non-GTA related mentions:
Rainbow Six: Las Vegas
Resistance
Infamous
Batman Arkham Asylum
Mirror's Edge
Mortal Kombat Komplete Edition
Tekken Tag Tournament 2
Tekken 6
The Last of Us
The Last of Us: Left Behind