FFTactics and Persona 3 Portable are my obvious choices, Disgaea is fun, but if you have the PS2 versions (or DS version) I don't really see the point. Locoroco and Half Minute Hero have demos available on the PSN network so I would suggest downloading those and trying them out first. Both are really fun but like Vyk said, their more for humor and passing the time as opposed to solid gaming.

I would also suggest KH: Birth by Sleep and MGS: Peace Walker unless you hate the franchises of course, you could also do what I did and mainly use your PSP as a portable PS1 by downloading the PS1 FFs, Xenogears, Legacy of Kain, and Wild ARMS, and putting them on your PSP's Memory stick.