If you can get your hands on an emulator, you owe it to yourself to try "The Castle" and "Castle Excellent", two of the best puzzle games ever created.
But do play them on an MSX; the NES version is nowhere near as good.
And Knightmare II: The Maze of Galious! Very good early Metroidvania game (only a year after Metroid) with a novel character-switching mechanic (later seen in The Lost Vikings, Trine and various Castlevania iterations).
But do play them on an MSX; the NES version is nowhere near as good.