Car: Chevrolet Impala SS 427 Convertible
What makes it special: For 1968, the SS 427 was a stand-alone option that could be had on the Chevrolet Impala Sport Coupe, Convertible, and the new-for-1968 Custom Coupe.
What made it famous: In 1968, the SS 427 offered a choice of the 385 hp L36 or 425 hp L72 Big-Block V8's. It was never featured in an ad, and it was given a small amount of space in the yearly catalog. (The L72 V8 option wasn't even listed in the catalog, but was on the dealer's order sheet ).
Why I would want one: Seeing that it was an off year for the once highly successful Chevrolet Impala SS, only 40,000 factory examples were produced that year. ( As compared to the 239,000 that were produced in 1965 ) This makes them fairly rare, and the fact that it's in a Convertible makes the "want" factor burn even hotter.
Fun fact: There is some controversy over whether the SS Impala's were available with hidden head-lamps. While this feature was introduced on the Caprice model for 1968 and wasn't listed as an Impala option, it was possible that a buyer could demand, pay for and receive that option on his car.
( 1968 Chevrolet Impala SS 428 Convertible photo from artvalue.com )