Metroid is definitely the hardest because you have no indication of how well you are doing, but Super Metroid is close after that. Super with no guide, some of the power ups are pretty much impossible to find, since the game breaks its own rules occasionally. Those white dots don't always mean there's an item, and sometimes they just aren't there altogether, so there's no indication of something to collect. Fusion and Zero mission largely addressed this, though Zero Mission has some incredibly hard shinesparks to pull off, and if you can't get 100% in Other M, well full completion may not be your forte, no offense.