There's one particular obscure NES game that comes to mind, called Little Samson. I never would have found out about it if not for sheer luck. One day (actually, one night at about 3am), I was stumbling through the depths of Youtube, my health running low and no bonfires to be found, when I ran into a video containing gameplay of this overlooked gem. I downloaded a ROM of the game (since cartridges now go for ridiculous amounts of money) and was struck by how just how good it was.
It's easily one of the best-looking games in the NES's library, and it features tight, challenging platforming gameplay with a few unique twists, the biggest of which being that you play as four characters that you can switch between at any time (an agile ninja that can throw things and cling to walls, an armored knight that can punch, withstand lots of damage, and walk across spikes, a dragon with limited flight and fire breath, and a mouse that can walk on walls and ceilings). It's not completely without faults (the level design falls flat in some areas, and one boss fight in particular is pretty badly designed), but the game is still good enough to stand with the best games on the NES, and I'm surprised that so few people know of it.