Here is the rarest game that I own for another day, Wizardry Knight of Diamonds for GBC. It's a great version, but I haven't played it in years so I might as well pass it on.
The Famicom version was already unique; this version shrinks it down from 20×20 to 15×15, making it feel like a new game in the process. It also allowed importing characters using a password from the other two GBC games. You didn't keep your level or equipment, but it did allow starting with a Ninja or Lord or whatever.
These GBC versions are pretty uncommon. Haven't decided what I'll do with the proceeds yet.