After hanging out with a retro games store for about 6 months now, I've learned a lot about what is considered rare and what isn't. Most rare games are usually the really fucking obscure titles that nobody in their right mind has heard of unless they avidly collect the games for that platform. Meanwhile the popular Zelda's and Pokemon tend to hold value, but aren't really considered rare due to how many of them exist in the world.
For example, a game I happen to have recently picked up at a yard sale (in loose condition) is a game called Spud's Adventure for gameboy. It's one of the first Atlus games (the people that now do Persona, SMT, etc.)
And it is worth -
Pretty insane huh? If you're ever curious as to how rare something you own is, you can usually tell by its value. You can check all that stuff
here, as it is a repository nearly all retailers use to determine pricing on retro and rare games.
As for the RAREST game I own, (inb4BanjoKazooiejoke) I own the Misadventure's of Tron Bonne on PS1.