God of War: Ascension
This game is easily the best showcase for the system, and it was a great gameplay experience as well. The only problem with the game was having to follow up the events of God of War 3. The book was already closed on this chapter of Kratos's life, and they tried to go back to the well one too many times.
When it comes to visuals though, this game is the culmination of every trick learned in developing for PS3.
Ascension is just incredible looking. Absolutely gorgeous game and not being tied down to the original trilogy structure allowed them to just go wild.