You have to do with your universe.
The rarest things can be you score, or the most common too.
If i take for example a post-apo French serie, the rare thing is cat food...
In Fallout it's bottle caps.
So... i dunno exactly how to help you, because the score is made from something the player has to achieve.
In my games, i often use time for score because i expect players to speedrun my games.
Think about what you want the player to do in your game, and what you want to use to compare players with each other
It's the best theorical piece of advice i can give to you.