Zelda II, the Adventure of Link. Downloaded on my Wii because I lost my GBA remake.
I'm a long-playing veteran of Zelda games. I've been playing for eighteen of my twenty-one years, and this game still presents a challenge. The Great Palace at the end is a literal maze, a huge, sprawling labyrinth filled with enemies that can put a major hurt on on you, such as those high jumping bird knights (the blue ones really piss me off) as well as those hopping...whatever they are that toss moving fireballs all over the floor, giant skullheads...well, you get the idea. The Great Palace is ruthlessly difficult, and is filled with dead ends and false-floor traps. You either need to map out your routes, or use Gamefaqs because you will get lost.