Zelda 2: The Adventure of Link. Beat it on original hardware for the first time maybe 10 years ago. It’s probably the hardest game I’ve ever beaten in my collection, especially those final few dungeons. It gets brutal hard near the end.
It took me years to beat. You just gotta keep grinding through it. The Japanese version is a little easier, because it lets you choose your stats when leveling up. In the American/Western releases, you could only level up what the game had selected for you. It’s called the black sheep in the series for a good reason. Oddly enough, it’s in my top 5 favorite Zelda games mainly for its difficulty.