Hmm, hard to say what my rarest game is. Living in Europe really adds to the rarity of a game, for example Chrono Trigger for SNES and Final Fantasy III (SNES) were never released in Europe. If you check eBay Germany for example you won't find these games for under 40€ (~$50), and that's for a loose game. It's not that much cheaper in the U.S but if you're lucky you could find them at a pawn shop or a garage sale. That's not going to happen here, whoever has that game and decides, for whatever stupid reason you could have to sell such a game, to actually sell it, knows what it's worth and knows that people will pay the price.
The three games I own that are rare no matter where you live are Castlevania Vampire's Kiss (aka Dracula X outisde of Europe) for SNES, The King Of Fighters 2002 for Dreamcast (since it was never released outside Japan for DC) and Mega Man: The Wily Wars for Genesis. From all the games I own theses are the rarest.
"Unfortunately" there are also some rare games on my wishlist which I plan to buy like Ikaruga (doesn't matter if it's the GC or DC game, they're both hard to get here), Mega Probotector (aka Contra: Hard Corps outside of Europe) for Genesis, Alien Soldier for Genesis, and my holy grail Cosmic Smash for DC to name a few.
As for my most prized games Secret Of Mana and Chrono Trigger for SNES are definitely up there. Secret Of Mana is to this day the only game that was so capivating that at one point in the story of that game, I was so deeply moved that I cried (ok, that was a over decade ago but even back then I didn't weep easily)
Another one is The King Of Fighters '94 for Neo Geo CD. I was used to Street Fighter II, played the hell out of that game, and with it became a Beat 'em up lover. I read all these features and previews of Neo Geo games which seemed to be like fighting game heaven for someone who was into that kind of games. One day I rented Fatal Fury Special for SNES and was amazed by the style of that game, same with Samurai Shodown for Genesis. They just felt and looked different from the Capcom fighters which at that point were just updated versions of Street Fighter II, and basically these two games were only watered down ports of even more beautiful Neo Geo games. When I read an article about a game called Survivor (which later became KoF '94), which was said to feature the most popular SNK characters from different games and with that a roster that was double the size of SF II's roster, I bought a Neo Geo CD (the AES was just too damn expensive for a 14-year old kid

) with KoF '94 and words cannot describe how amazed I was. The characters, the backgrounds, the sheer style and overwhelming detail of that game left me speechless. I have played all KoF episodes and '94 is not even the best (that would be 2002 in my mind) but it holds a special place (and the stage of Team Brazil has one of the most beautiful backgrounds I have ever seen.
The last ones that come to my mind immediately are Weapon Lord, Unirally (Uniracers outside Europe) and Zombies for SNES. These games were just unique in design and just so much fun. While Weapon Lord was "just" another Beat 'em up it was "different", it had charismatic fighters, beautiful stages and sophisticated (and at that time unparalleled) weapon combat. I consider that game Soul Calibur 0.5

I still play these and if they hit the Wii's Virtual Console some day I'll buy them in a heartbeat despite owning the carts already.
X-Com: Terror from the deep for MS-DOS is the only PC game that I own and I was addicted to this game. Still am when I think about it

It just had such a deep (no pun intended) gameplay and it was hard but at no time frustrating. I don't even know how often I would have to load a save game because I messed up or something unexpected happened. The "tycoon" part of that game was great and extensive and the combat missions were captivating and had a very eerie atmosphere. I would love if this game would get a 2nd run on the DS. Rebelstar Tactical Command for GBA was so great, I can't imagine what X-com for DS would be like.
That's it from me. It was long and boring but you said the more details I gave, the better. So don't blame me
