[quote name='Josef']The rare Super Mario Bros / Duck Hunt variant where both games are combined on to one NES cartridge.
That and maybe Stadium Events for the NES.[/quote]
lol
So, no one has listed the rarest game ever, I see.
Powefest 94 is the rarest game ever made. Only one copy exists, and every now and then trades hands for around 100,000 USD. It was a special competition SNES cartridge made for the Powerfest 94 videogame competition. 33 were made, and 32 were stripped down for parts.
After that, it's the Gold Nintendo World Championships cart
And third place goes to the already mentioned uber rare neo geo cart, Kizuna Encounter. For those who do not know, the Neo Geo was a home version of SNK's extremely popular MVS arcade format. Strangely, the Kizuna Encounter carts that are MVS are rather common.
There's a lot more too, but I found out about most of them from here:
http://www.racketboy.com/guide/the-rarest-and-most-valuable-video-games
I'm not advertising the site, it's just a really good resource if you want to know what to look for when you're out hunting for games. They have very good lists of rare, overlooked, and cheap games
Here's the wikipedia article about POwerfest 94:
http://en.wikipedia.org/wiki/Nintendo_PowerFest_'94
And here's a picture of the ridiculous looking cartridge:
http://images.google.com/imgres?img...+94&hl=en&sa=N&um=1&ei=hrMDSrmCGomEtgPt1cCCAg