That God... damned... Goron challenge in Majora's Mask. Seriously, still haven't taken the hours out of my day that someone as incompitent as me to do it.
Completing Jet Force Gemini. Damn, that game got pretty damn hard by the end. And when you beat the "final" level, sudden plot twist and the game basically triples its length. The third boss, which one would have believed to be the penultimate boss, is pretty damn intense. Twin robot mantis things that shoot freaking missiles at you and take like six of your missiles each to break their rocket-firing arms. And then you have to kill them.
After that, you finally complete what would have been the final level, and then beat the final boss in thirty seconds. But wait! He survives and he escapes, and to catch him you need a super-fast spaceship that your friend seems to conveniently have. Then he tells you parts of it are missing. Then he also tells you he's not going to let you fly the thing until you rescue all of his people. All of them, from everywhere, even planets you haven't yet discovered. Essentially, the game is forcing you to explore everything and backtrack to find things you might have missed, and also reach previously inaccessible areas. And to obtain some key items you have to do things that are seemingly only extra minigames (oh god those Floyd missions, and they required you to get the gold medal in one of them).
Then after essentially 100%ing the game, you reach the final level. And then the final boss, for real this time. And this time he's fifty times harder. First he launches a volley of meteorites at you. Yeah, that's the first thing he does. Then he shoots freaking eye lasers and hand lightning at you, and they do ridiculous amounts of damage. After he attacks you about three times, there's a tiny window of opportunity for you to shoot a small area on his back while he's constantly moving. Hit him, then he repeats his pattern, except each time the length of time when he turns around gets shorter and shorter.
Still a great game though. Oh yeah, also that mission in Club House Games in Bowling where you have to hit a single pin five times in a row. And I still haven't managed to get a Backgammon Win ever.
May we meet again outside the battlefield.