Hypermode on Prime 3 is easily Metroid's most difficult moment. But as you said, it was extremely satisfying to beat it. Other M's hard mode was missing a little something to give it the feel that Hypermode has. You didn't really feel as badass in Hard Mode, possibly because Hypermode was actually the game throwing everything it had at you, trying actively to make you fail, and making it really feel like this was Dark Samus showing her hand, making her final assault on the galaxy in an attempt to conquer it. This is how the hardest difficulty in a game should be, it should be the developers designing the game so that it's actively working to make you fail, rather than many games these days handing you a victory on a silver platter.