Killzone for me.
It's absolutely gorgeous in visual and sound design and is the best way to show off the new hardware, at present. I'm really enjoying the campaign (the recent patch seems to have sorted out many of the annoyances), which I'm towards the end of. I can't wait to get my teeth into multiplayer, which I'm sure I'll love.
Going by the Metacritic score, it is very underrated. I'd highly recommend it.