CASTLEVANIA 3 / Konami / NES
Castlevania was pretty good ... Castlevania 2 not so good ... but Castlevania 3 was really where the series became the kickass enterprise that it is today (er, used to be anyway ... before Konami farmed it out to their B squads, Animu'd it up for cheap profits, and burned out Michiru Yamane the same way Squaresoft ran Nobuo Uematsu into the ground). 20 years later, this game might not look like all that much, but upon release it was really quite the graphical and aural tour de force for the NES, and it also happens to be one of the paragons of Badass Platforming.
Even better is the Japanese release, Akumajo Densetsu, which was translated by Vice Translations. This release added an external sound chip with three extra channels and an extra graphics chip that provides for some effects in the background in some levels that weren't present in the English releases. Also has uncensored crosses and all that, plus you're now in Wallachia gallivanting around as Ralph Belmondo.
The game is pretty nicely balanced, and challenging without being ridiculous. There are some sticky points - it's way too easy to accidentally fall off of stairs to your death when trying to mount them, and on the flip side most of the bosses are too slow and predictable. Minor issues in an otherwise badass game, however.
* Gameplay Video