Disgaea 2 looks great, but it looked great on the PS2 as well and scaling down the viewable area makes everything look crisper. Environments are detailed, character models are great, and the anime cutscenes (static, sadly, but that is not a downgrade from the original) look wonderful on the PSP screen. The text is clear and readable, too. You really couldn't ask for a better visual treatment.
If you can only afford to buy one PSP game, then make it Disgaea 2: Dark Hero Days. It will be a good long time before you run out of things to do, and you'll be having fun all along the way.