Lumines: ES, Guacamelee!, Gravity Rush, Sound Shapes, Little Big Planet Vita and Persona 4: Golden are the standouts. You should want to own at least two or three of these, and if none of them appeal to you, then the Vita probably isn't your system. (two of these games are indeed available on the PS3, but were originally designed for the Vita, which shows)
Here is a list of Vita games which you should look over- again, if nothing stands out, its probably not for you (I own and would recommend well over a dozen). Keep in mind the Vita also has backwards compatibility with the PSP games you buy off the store, as well as a great deal of PSone games (with some notable exceptions like Crash and Spyro). So you're looking at a wide range of games to play, but maybe not a whole lot you couldn't play anywhere else.
A couple notes in no particular order:
- the Vita ecosystem relies heavily on downloadable games. if you don't plan on downloading games (like, say Hotline Miami or Mutant Blobs) you'd be doing yourself and your Vita a terrible injustice by skipping over a lot of its support.
- the 8gb memory card is the "sweet spot" if you are thinking of a mix between physical (cards) and digital. you won't be able to have all of your games on you at all the time, but you will be able to keep all of your game saves handy, and downloading your games back off the PSN is painless. I regularly have a folder full of 6-8 games I swap in and out depending on what I'm in the mood for.
- Playstation Plus will save you a lot of money, between regular sales/discounts and free games. It also has a bunch of handy features like hosting your saves on "the cloud" as backups. If you already have it you'll be picking up your Vita with 3 of the best games off the bat.
I've owned every handheld released in the US short of the Virtual Boy, and the Vita is my absolute favorite to
use. It feels great in your hands and has a lot of features, like (free!) party chat and custom soundtrack support, that should be mandatory from now on.
It doesn't have as many need to own games as the 3DS, or the breadth of cheap usability the iOS ecosystem has, but it does its own thing and it does it
amazingly well. Just keep that in mind if you're seriously considering one.