I think it was Resistance: Burning Skies, unfortunately. I have 8 Vita Platinums total and that was easily the worst of them (the other games being Uncharted, Gravity Rush, Sound Shapes, Persona 4 Golden, Playstation All-Stars Battle Royale, Plants vs. Zombies, and Guacamelee)
Oh man ... you did Gravity Rush? I couldn't bear to go through all of the challenges.
My first was Unit 13...since then I've gotten Shinobido, Resistance, PVZ, Atelier Totori and LBPV.