I assume TC excluded Zelda II and MM on purpose, seeing as they're typically cited as the hardest, and wanted to hear of something else.
I would say the original Zelda. It seems darn near impossible to beat without a guide, and even if you know what you need to do, it can still be hard to actually do it.
Not that that's a bad thing, mind you. Personally, I was Nintendo would ramp up the difficulty in their new games, not make them easier.