Aside from the chromeline, stealth, and academy cars, nothing is rare. I guess you could say the X2010 is, but everyone has one, and everyone gets one by default when they hit a specific level.
I figure the stealth SLS is the rarest car since the only way to get it is buying the signature edition(or living in japan, they get everything).
Every other car, be it premium or standard is not rare. The UCD is not random and you can calculate when you will find specific cars, douping of old has given most people any expensive car they want.
http://www.mygranturismo.net/carlist.php?order=rarity&start=0 has a surveyed calculation on what cars are rare. The list is flawed, since its calculated by visitors listing the cars they own and the site figuring out what car is listed the least, but thats not accurate because not everyone lists every single car, and many dont even bother to spend hours listing any cars at all, not to mention, undesirable cars that people simply dont buy would be considered rare despite being available in the UCD all the time. For example, the top car on its rarity list is the ford GT 02. I have 4 of them, see it all the time, not rare at all.
short answer is there are really no rare cars. chromeline, and stealth are the only real rare cars since its impossible to get them without purchasing the specific edition of the game.
