What is the hardest game in the Metroid series? Is the original? Maybe even the remake, Metroid: Zero Mission? Or even the light vs. dark game Metroid Prime 2: Echoes? Or maybe (dare I say it) the pinball remake of prime Metroid Prime: Pinball?
I would say the original or Return of Samus. The games were both tough, but I'd have to say that RoS is tougher due to the technical limits of the game boy. It made it hard to play all around.