You want to know which is the rarest car with the rarest paint color that you don't see around here?
Lots of people have the expensive race cars and "hard to get" cars but not a lot of people have the cheapo stuff. You don't see people asking for really any of the 100 Miatas in the game, do you? They are probably the hardest to come by.
My opinion, finding a car with 0/0, I am having trouble finding people to trade their RUF 3400S to me.
I'd say look though my garage. I have all the cars in the game and there is a rarity rating along with each car. As you will see, a lot of the race cars everyone has but a lot of the cheap cars, next to no one has.