Super Metroid for me due to sequence breaking and tons of advanced techniques that were unintended imo.
SOTN is way too easy even if you aren't trying. Yeah you can somewhat sequence break it but not really due to the items you need. You have to limit yourself to low level if you want a challenge due to Alucard being miles above all the enemies (magic, turning into bat, multiple weapons and items). Also love the fact when you die you have to watch the death scene over and over which is unskippable. Faster to just reset lol.
Both are great games though. I just wish Symphony ditched the RPG elements and went full Castlevania with metroid exploration elements. Imagine a version that instead of health potions they balanced it with health upgrades. I never really liked the leveling system. Many times you just brute force shit with levels or if you aren't leveled die in one hit. Both extremes are bad.
Essentially what Richter mode is but without feeling so clunky. You can definitely tell some of the enemies do not mesh well with him. Probably why they gave him that up that up, down to forward dash attack with invincibility frames to remedy this. Feels a little awkward imo.