Adventure

I never had an Atari 2600 while growing up, though I had friends who did. (Years later, I eventually bought one as an adult, almost as an impulse.) On those occasions I was able to play, I often found myself strangely attracted to Adventure. Sure, the graphics were unsophisticated (even then) and the dragons seemed to resemble some sort of duck ― but somehow, the simplicity imposed by the limitations of the system became the game's strength. And it could be quite a challenge:

The player's goal is simply to find the enchanted chalice and return it to the gold castle. The player character, represented by a square, explores a multi-screen landscape containing castles, mazes, and various rooms. Hidden throughout the world are a sword, keys to unlock each of the three castles (gold, black, and white), a magic bridge which allows the player to travel through a wall, and a magnet which will pull any of these objects toward it.

Adventure has just been brought to the iPhone, just as I remember it ― and it's free! (And I'm no better at it than I was the last time I played, which must have been 20 years ago.)