Oh this is a tough one.
For me, it's a toss up between Ace Combat: Skies of Deception and Patapon 2. Ace Combat: SoD kept me hooked for hours upon hours just flying around doing missions. Ace mode was a brilliant test as well, one that I enjoyed to the very end. With Patapon 2, though...I kind of spent 40 or so just playing the demo alone. The game was that addicting. I've logged in maybe 100 total hours on it now, and I'm still finding new ways to play the game. It's truly an experience, and the best of that respective series.
Honorable mention to Killzone: Liberation, which is easily one of my favorite shooters on the system. I loved the story and the gameplay was awesome.