D-Pad Hero (http://dpadhero.com/Home.html), a unique twist on the rhythm game, now for NES (emulators)!
Probably the most difficult NES game made to date. The article I found about this (http://raddevon.com/d-pad-hero-rocks-it-old-school/) said "it's harder than if Mega Man 9 and Ninja Gaiden Black consummated and gave birth". They're right.
I doubt it's harder than the immortal classic, I Wanna Be The Guy.
Or La-Mulana's aptly named Hell Temple.