I'd agree with Nokkon about God of War, it's an essential PSP purchase.
I picked up Daxter months ago, but didn't get into it.*
I'm currently playing through Ratchet and Clank - Size Matters, and enjoying it... was cheap too
I also picked up Space Invaders Extreme last week, which I'm finding particularly addictive.
I also love the Pursuit Force games on the PSP.
Vin.
* I'll give it another try after Ratchet and Clank... I bought both at the same time, and because I liked the PS3 R&C game so much, I didn't really give Daxter much of a chance.
EDIT: and Patapon is just far more fun that it should be

Note, if you import it, it's half the price (or less) of UK prices.
EDIT2: and I'm loving Burnout Dominator on the PSP, so that's one to look out for if you like Burnout games (and is also cheap these days)