Perhaps one could make the argument for one of the anticipated "4s:" Uncharted 4 , Killzone 4 , or God of War IV .
But in reality, I think the game that might showcase the new PlayStation's talents best is Gran Turismo 6 . Obviously, there's no guarantee GT6 shows up for the PS4's launch but Sony has put a premium on quality launch software recently (that's why they held up the Vita launch), and they understand the importance of having AAA must-have games available when a new piece of hardware hits the market. It's just so important.
And considering the projected PS4 launch date (possibly late 2013), GT6 might not be entirely out of the realm of possibility. Also, let's not forget that Gran Turismo 3: A-Spec was the first true-blue system seller for the PS2; this is a franchise that has sold around 68 million overall and GT5 has passed 9 million. That's pretty damn amazing for an exclusive title, especially considering that GT5 has essentially sold what the last two Uncharted games have sold combined (or thereabouts).
Lastly, the technical prowess required to truly fit Kazunori Yamauchi's vision is staggering. He has even admitted that in the past. GT5 pushes the PS3 and there's no doubt that GT6 could really take advantage of the PS4's expected power, especially if we're going to more accurately include vehicle damage and other matters of depth. If GT6 releases with the PS4, you'll really see some fireworks…