Thought I'd sneak in the 1802, but Rob beat me by a couple mins.
Considering rarest as survivors in 2013, then you may be correct with the ti. Some 16,000 were produced but I guess many were raced, ridden and put up wet and ended up recycled steel. Certainly there aren't many known in the US (maybe more in Europe?). The Turbos, Targas and Cabrios seem to have been looked after, and survive eventhough few were made.