"Most Original" is a bit of a misnomer. If something's original, then something else can't be more original, it can only be original in its own way. I mean, Pong was original, and if you wanted to create enough of an argument, you can say nothing else is original because Pong used the video game idea first.
Sorry, I'm being nit-picky.
Anyway.
Given to answering what I believe as one of the best original concepts, it would be Metroid Prime. The ideas in it were all used before quite well, but having Metroid from a first-person perspective ended up being my favourite of all Gamecube games. This is mainly because no other shooter I've played gave me the rich lore of the Chozo or the ample information on the Sheegoths of the Phendrana Drifts. I never felt like the lore was replicated as well in the sequels, which were nevertheless very good.