I bought a used PSP and the remake of Final Fantasy Tactics years ago because I loved FFT as a kid and just had to see the remade version. the re-translation (if you want to call it that) was cool and all but the graphical slowdown during spell effects made it more or less unplayable.
BUT NOW I'm armed with a $25 gift card to Gamestop and some junk to trade in so tell me. What's the one PSP game I simply cannot afford to pass up?