N+ is possibly my favorite platformer on the PSP, and it sold a measly 30k.
Both of the Classic Dungeon games were amazing. Just thinking about them makes me want to go back and play them. If you're a fan of action RPGs, Dungeon Crawlers, and classic games, you can't go wrong with these games. The soundtracks are amazing as well, and they even give you the choice between 8-bit and modern music. Cladun X2 does surpass the first game in every way, so I recommmend you get that if you're interested.
Locoroco: Midnight Carnival is my favorite game in the series. The reviewers panned the game for being too difficult for the controls, but I didn't have much trouble after the first few, easy, stages.