I have several extremely rare NES games. Even though some are incredibly rare, they aren't worth a whole lot (relatively) compared to a lot of more common games due to collectors' lack of knowledge or desire to collect them.
Some of my more cherished gems:
Probably the rarest licensed game ever
I have all 4 variations of this
Papillon Gals
No online image for this game. Supposedly only a couple full NTSC copies are known to exist. Obtained directly from the company in Taiwan prior to them shutting down.
One of the 150 promotional copies of this
I have every year of these, with 2008 being the rarest. This is a homebrew release but highly sought after and collectable. Only ~50 of these released.
I've had a few prototypes. Not super interested in those, but I guess they are unique. Would only be super interested if it were an unreleased prototype of a game that was super special to me as a kid.