I've had a lot of rarities walk in my UCD (I started out with the Nomad Diablo in it, and almost threw my controller across the room in rage), but I was praising the lord when I got a Plymouth Cuda 440 Six Pack. I plan on putting a silver chrome chip on it and savoring the pure display of muscle.
Also, earlier today, a Corvette Z06 Racecar (C2) was in my UCD, but I was level 19 and needed about 200,000 more credits to get it. I went and did the NASCAR Seasonal Event, and used the max amount of times I could go back to the race menu before the C2 would be cycled out of the UCD. I scraped together a 5th place finish, and earned just enough exp. and cr. to rank up and buy it.