The rarest car I've seen is the 2002 Ford GT (yellow w/ black stripes). Just under a million credits.
I've seen it once in well over 1000 "days".
I've seen every other car in this thread multiple times.
I bought one today from my UCD. I had to sell like 10 cars to get the 1M for it though as I only had 600k at couldn't do any races otherwise it would go. This is one car I am not going to trade, even by duping. I want it to remain rare for as long as possible.