It's a toss-up between Zelda II's Dark Link and ALttP Ganon for me. The fights seem pretty simple but you can really take some serious damage quickly. Dark Link is probably physically harder but I feel like ALttP Ganon is more stressful.
Not to mention that he can tear through shields if you aren't careful. If I hadn't taken the extra time to level up one of mine, I would have been very dead. Otherwise, he isn't that bad.The first thing is just that he does a lot of damage, iirc. Besides that, his attack patterns can be a bit tricky if you're not used to them: the counterattack he does after hitting his sword a few times, the two swings in a row, etc, can result in you getting hit a lot if you're not careful. In the second phase especially, I didn't really get how to call the lightning quickly and kept getting hit before I could.
The fight can be incredibly easy if you actually know what you're doing, of course. But it gave me some trouble the first couple of times until I got it down.