Persona 4 Golden is IMO the number one Vita game currently but Zero Escape: Virtue's Last Reward, Metal Gear Solid HD and Gravity Rush are all awesome too. I'm sure there are alot of good games I'm missing aswell.
I own handhelds and love them both. However I tend to use my Vita more especially when paired with Playstation Plus. SCEE has given us some great Vita games for free in past months.