I found Killzone a little awkward to control, since the Vita is bigger than a controller and doesn't have triggers.
Uncharted is the weakest of the series and all the touch screen stuff feels tacked on for the sake of it. Rather than really adding anything, since all you do is clean stuff. There are a couple of puzzles that use the touch screen, but that's about it.
Tearaway is very original in parts and uses the touch screens far better than any game I've played so far, but I got bored after a couple of hours.
Final Fantasy X I'd recommend if you're a fan of FF games, but the voice acting is poor and some of the save points are too far apart. Though it's not too much of an issue on the Vita because the game stays "open" the same way as apps do on your phone.
I've just completed Muramasa Rebirth and thought it was excellent, a beautiful 2D graphics, controls fantastically and feels perfect as a hand held game. The only negatives is you could say it was a bit repetitive and some of the music is a bit too oriental for my taste. And no voice translations, only subtitles.
Resogun is a great twin stick shooter and buying it once gets it across PS4, PS3 and Vita platforms. Oddworld New nTasty is also crossbuy and is a great fit for a handheld.