I'd like to say Xenoblade, but a 92 on Metacritic is good enough. I'd also like to say Virtue's Last Reward, but GameInformer just gave it the "Best Hendheld Exclusive" award yesterday, so it's getting proper recognition.
Kid Icarus is really fun and I dumped dozens of hours into its addicting (albiet a little repetitive) online multiplayer. The Vita-equivalent in terms of underratedness might be Gravity Rush, which I will try whenever I finally get my hands on the Vita. Probably sometime next year. Definitlely if it gets a bona fide price drop. Absolutely with a Sly 4 bundle (not gonna happen).
3DS Friend Code: 0645 - 5827 - 5788
WayForward Kickstarter is best kickstarter: http://www.kickstarter.com/projects/1236620800/shantae-half-genie-hero
