Rarest to me is synonymous with OEM, nothing aftermarket. I grew up seeing the muscle cars of the late 60's/early 70's sit on dealer lots for next to nothing in the mid to late 70's after the oil embargo of 1973 and following "gas crisis", emission regs, etc. Now those cars are going for upwards of six figures, the most valuable being the one's that remain stock and with low production options.