For the seen-it-all... Killzone: Liberation (read our review)
Given that it's a top-down, seemingly stripped-down sequel to a poorly received first-person shooter, even the most jaded gamers could be forgiven for thinking Killzone: Liberation is worth ignoring. Too bad they'd be missing one of the most intense, engaging action games on Sony's handheld. Liberation tells the story of a war in the far future, but that's really just window dressing for gritty, tactical action that constantly challenges the player to stay alive against overwhelming odds.
It's difficult as all hell, but in the best way possible. If you've got the type of blasé gamers on your hands who plow straight through everything else, then Killzone: Liberation will be more than enough to keep them glued to their PSPs.
Second choice: LocoRoco