the only truely hard metroid game is the original imo
II is tricky as it has no map, but i wouldnt call it hard
Super is perfect, not easy, not hard
Prime is perfect, not easy, not hard
Fusion is hard in places (the whole running away from SA-X and the Spider boss), but the rest is generally easy
Prime 2, its easy to get lost but again i wouldn't call it hard. i never "got" the difficulty of the boost or spider guardians
Zero Mission is childs play
Prime 3 is easy