Ghosts and Goblins
I'm not saying Ghosts n' Goblins is easy. Out of hundreds of NES games, it's right at home in the top 25 and for good reasons. The enemies are brutal, particularly the red devils who require precise timing in order to defeat. You only get what amounts to a single hit point, and suits of armor (which grant you an extra hit point) are few and far between. The level designs are demanding with their precision jumps over chasms and onto moving platforms and the incessant barrage of enemies. Enemy placement is one of the games most devious aspects. There is always a tattooed thug on the ladder just above or below you, a blue bird always flying right into the path of your jump, or a red devil perched on a cornice that will attack as soon as your foot touches the ledge. Oh, and every now and then, you'll be turned into a frog. The game insists that you have quick reflexes, a sharp eye, and nerves of steel.
You'll need all of those to beat Ghosts n' Goblins, but you'll be armed with those unlimited continues I mentioned. Also working in your favor is the fact that most of the levels are extremely short. Compared to other platforming games, the levels in Ghosts n' Goblins pass by pretty quickly once you get to know them. GnG experts can probably clear a level with no deaths in about 2-3 minutes. All told, this means that once you are a master, the game takes about a half an hour to beat each quest. And once you get really rolling and have the levels memorized and all of your strategies plotted out, you can roll through this game with relative ease and have a lot of fun doing it.