So far I've seen people mention games you can discover with an easy Google search. They're games EVERYONE knows about... So I'm gonna try to let you in on a few hidden gems...
Note: The name of each game on the list is a link to a YouTube video about the game.
1.
Holy Diver:Holy Diver is a Famicom exclusive. It's basically a cross between Mega Man and Castlevania. It's easily the hardest game I have ever played, even moreso than Battle Toads (which, if you do not know to pick that one up, I suggest picking it up). You will need a converter to play it, though. There's plenty of videos online telling you how to get a hold of one and make it yourself. It's very easy.
Here is an example of such videos. Many "Black Box" NES games have the converter in them. There are several indications where it may have a converter. Just Google that as well.
2.
Recca: Summer Carinval '92:Easily the best Shmup on the NES. It was created for a tournament held on July 14th, 1994 in Japan. Since the Famicom was a dying breed, not many cartridges were produced. The original Famicom cartridge goes for $100 - $150 on eBay, but you can get it on a reproduction cart at The NES Dump
here. You can also find a reproduction of Holy Diver under the "Translated" section of the site as well.
3.
Battle Kid: The Fortress of Peril:
This is a homebrew, an entirely new game developed by Sivak Games. It is available at
RetroUSB.com (also known as Retro Zone). It's inspired by I Wanna Be The Guy.
4.
Dragon Fighter:
Dragon Fighter is a very good action game for the NES.
5.
Metal Storm:
Metal Storm is an NES game that really should've become one of the most memorable games in history. Even IGN and other big-name video game websites somehow overlook this masterpiece. It's an amazing game that you should get NOW.
6.
The Krion Conquest:
The Krion Conquest is a Mega Man-esque game... though it's much more difficult. Very fun and stylish and you have plenty of powerups you need to learn to utilize from the very beginning.
I'm done for now! Perhaps I'll suggest more later, I could go on for a while!