It's a toss up between Uncharted 2/3, God of War 3, and Killzone 3. Other really good looking games that are hard to ignore in terms of graphics are Motorstorm Pacific Rift, Gran Turismo 5, and MLB 12 The Show.
But basically, most if not all of the exclusive PS3 games look fantastic. So if it's exclusive then you can't go wrong (with a few exceptions like Haze)
-Why don't you wait until you're asked? -Why don't you ask me?