Metroid Prime 2. People tend to call the game punishing, which I find humorous given that they want their Metroid to be dark and atmospheric (but easy too apparently).
I appreciate what they tried to do and still liked the game in spite of it, but damn the dark aether was no fun at all. My favorite things from Prime 1 was exploring pretty environments at my own pace and getting excited to backtrack and try my new power ups in previous areas, but the sequel has half of the game look the same all while draining your HP constantly. It sucked, I think it's the only Metroid game I've liked that I don't really want to replay ever again because of that.
Ironically tho, it has my favorite escape sequence in the series, with a cool remix and all. So cool.