Definitely the most challenging Zelda
Very, very challenging game. The side-scrolling parts make things much more difficult, especially later in the game; although, by dungeon #2, things have gotten pretty hard. This made for a very enjoyable game, especially when overcoming obstacles and accomplishing various objectives (getting items, raising levels, finishing underworld dungeons).
Some parts of the game were just TOO difficult (i.e., Death Mountain -- getting the hammer...took me at least a week). The final dungeon (end of the game) is also very hard. By the time you get to the Thunderbird, you'll be lucky to have any life and magic left. Good luck beating him. I was never able to finish this game on the NES. It took me about 15 years from the time I bought it until the time I used cheat codes on a NES emulator to finish it. Using the original NES controller was almost impossible for this game. The NES Max helped a lot. The music is this game was also a little sub-par. Zelda 1's music was better, I thought.
The Bottom Line
A great sequel for Zelda 1. Play that game first (play it to death, since that is one of the greatest video games of all time), then get a hold of this. A step-up in challenge and graphics. Classic game.