I'd say the original Fallout has to be one of the darkest RPGs out there. Think about it: you're cast out from your comfortable womb of a world into a harsh reality that can kill you at any time, you start out with nothing but your Vault suit and a knife which in Fallout terms you might as well be naked...and you still haven't even left the cave.

Oh and you have a time limit. Good luck :D.

If you want a dark RPG, go with the original Fallout.