PSP games that I liked that are unpopular...hmmm?
If you haven't installed any emulators on your psp and you're actually playing on an actual unit or a Vita, I'd recommend Capcom Classics Reloaded on PSP. I liked it because it had the Ghosts and Goblins trilogy, even Super Ghosts and Ghouls which szx an SNES game as well as the arcade originals. Also has all the Street Fighter 2 games all the way to SF2T.
And on that note, you can add Sega Genesis Çollection. It has most of the games from other similar collections including Phantasy Star 2-4, handy games if you're on the train and you can just grind and plod on with your party.
Finally, there's Pinball Hall of Fame: The Williams Collection. Another collection great for passing the time while stuck behind a queue which I did by playing short games based on real life pinball machines.