I imagine it may be my copy of Bubble Bobble for the NES, but I have no idea how rare that is. Either that or Diablo on PSX, or Pokemon Stadium GS for the N64. I've never given it any thought how rare any of my games are, but my collection's not that huge at the moment so it doesn't take me long to think about what's in there :P
On a side note, I do own something most residents of the UK and North America will never have seen in person, but the rest of Europe would just shrug: my French and German, respectively, copies of FF8 and FF7 on PSX that I bought during my A-levels. Huge help, they were, despite being the most bizarre translations I have ever seen. French FF8 seems to have been translated straight from Japanese; either that or they looked at the English version then tried really hard to forget it... the scene where Ward loses his voice? No, he takes a vow of silence instead. I'm not joking. Oh, and loads of items that had English names in the English version have different English names in the French version, including a loot item called "9-inch nail" that the game describes as the "claw of an industrial monster." Worldwide punomenon.
http://www.angelfire.com/rpg/supponibarodu/Yeah so SpoonyBard eventually did a new Final Fallacy game.