Graphically there aren't any titles that "really showcase the systems true potential" yet. We're still in year one, so the majority of what we have is a functionally linear jump from PS3/360 tech. inFamous:SS is probably your best bet in that regard (also a very fun game, if a little lacking in content), but even that's mostly built upon 7th gen tech.
It will probably be another year or so before we see the PS4 and X1 really come into their own, but once the ball is rolling things should progress very quickly (rather than the slow trickle we saw in the 7th gen).
Anyway, I'd recommend giving Shadow of Mordor a try. Its visuals won't blow your socks off, but it's still a really great game.