The Dementium series is very underappreciated. They are a pair of first-person survival horror games for the DS. Contrary to their platform, they manage to actually be pretty unnerving most of the time, but more importantly, they're fun as hell. The game plays like a metroid-vania sort of game, but seperated into 5 different chapters. Throughout these chapters, you solve puzzles, fight monsters, and gain new equipment to prepare for the final showdown with Malatesta.
My personal favorite part of the game comes at the second-to-last boss, a huge colossus of tumorous flesh that chases you through a narrow corridor. You have to throw dynamite in its mouth to kill it, while jumping and ducking through obstacles. It's a fast and frantic boss.
I just wanted to get the series more recognition, because it really deserves some.
Here's a gameplay trailer for the second one.And a slightly more disturbing one. 
