That's really hard to answer.
The consensus Top 5 games are probably (in no particular order):
Uncharted: Golden Abyss
Killzone: Mercenary
Unit 13
Gravity Rush
Tearaway
Gravity Rush and Tearawya are far, faaarrr from the typical video games. Refreshing experiences from what I've heard. Sony made four of these games, Ubisoft the fifth (Unit 13).
I own four and enjoy 3 of them.
Uncharted, Killzone, and Unit 13 are the three I enjoy.
I'd like to touch a bit on Killzone in particular:
Everyone owes it to themselves to play the game if they are a Vita owner, even if they don't like shooters. I don't necessarily despise shooters, but I am in no way inclined to play them nor do I enjoy most of them. I hadn't played them in years and due to the insistence of people I knew I bought BO2 and consider one of the greatest wastes of money I've ever made.
Even after that bad experience I went ahead and bought KZ:M, partially to support the Vita I enjoy so much, but moreso because the consensus was that this was an amazing game, which wasn't the push ehind BO2 so it wasn't a 1-1 situation. Plus, with a pricetag of $9 I couldn't say no.
As it turns out, I don't regret the decision! I got more than my money's worth just from the campaign, it had an excellent story and was decently long, but nothing like RPG length obviously. I don't even play the multiplayer often, something that adds an immense amount of value to the game, unless its with
/r/VitaGameNight (Reddit Sub) or more personal friends. There's even a botzone so you could take 'multiplayer' on the road and on the go with you offline.
It was without any doubt built from the ground up with the Vita in mind and is the definitive shooter on any handheld, a game that is considered by many better than its PS4 counterpart.
----
Gravity Rush is the game I didn't enjoy, which isn't to say I dislike it. I really don't have an opinion of it. Reason? I didn't really play it much. I got it and couldn't get the controls to work for me, so I gave up sort of quickly. I'm sure I'll go back to it eventually, but I have so many other Vita games to enjoy I'm in no rush.