SHMTN3 gets my vote. I feel its a underrated RPG which is pretty damn unique.
A basic summary of the the game is as follows.
In Shin Megami Tensei III: Nocturne, you are a boy who starts out by taking an innocent trip to the hospital to visit your sick teacher, Yuko Takao, with your friends Chiaki Tachibana and Isamu Nitta. On the way to the hospital and upon arrival, there is a strange feeling in the air accompanied with even stranger happenings, and soon you find yourself the survivor of the apocalypse and a resident of what is called the Vortex World. Given amazing powers and capabilities bestowed upon you from the Magatama given to you by Lucifer, you must set out on a quest to help recreate the world how it once was, or give birth to a new world with the guidance of a Reason.
The game gives you a lot of customization over what you do. Your decisions, and how you choose to shape the world is in your hands. Your stats when you level up are added by you yourself. Your abilities that you wish to learn are up to you, and probably the most fun feature of the game, your party is up to you. Through out the game you fight demons in your basic turn based gameplay, but these demons you fight can join your party during battle if you wish. Throughout the whole game your party is made up of the demons you recruit. You can also fuse two demons in your party, and create a new powerful demon. Some bosses can even be recruited at a later point. Graphic i found great for its time. The story is damn fine also, lots of surprises and very open depending on what you decide. Due to multiple endings, rebuilding your character differently, and creating a whole new party setup. This game has tons of replay value.
Although this RPG might be a bit pricey since its rare, if you can find it at your local gamestop/gamestore bargain bin i highly recommend you pick it up.