Disgaea 3 - if you haven't played the PS3 version, or if you just haven't played it in a while it's an extremely deep and quirky (read: Japanese) grid based RPG.
Gravity Rush - It's controls take some getting used to, but it's a good experience once you do, and it was recently named Game of the Year at TGS somehow... It also wins points for being a Vita unique experience, as so far the Vita kinda has the same problem the PSP did for most of its life - almost all of the games are ports or spin offs of console games.
Lumines - Solid puzzle experience, with really good music
Ragnarok Odyssey (upcoming) - Seems like a solid, PSO/Monster Hunterish experience. Comes out next month.
Persona 4: The Golden (upcoming) - Obviously, the same situation applies to this as Disgaea 3, but there are a ton more goodies in this version.
PSX Classics - Yeah, I would recommend these too, just cause it can be nice to have them in a portable form, and the Vita recently gained the ability to play them (but not all of them). Notable ones available off the top of my head are the FFs, Alundra, Grandia, and Vanguard Bandits (I like RPGs
)
So yeah, that's what I can think of at the moment.
