[L78Impala]

This is my favorite subject.
As per the "Chevrolet SS Muscle Car RED BOOK ",
in 1965 there were 1,838 L78 RPO's ordered in the big car. ( 396 x 425hp) I know you were asking about L72's, but I had to throw that in.
In 1966 there were 1,856 L72 big cars ordered.
In 1967 no L72 cars produced. (17 L72 engines were built???)
In 1968 there were 568 L72 big cars ordered.
In 1969 there were 546 L72 big cars ordered.

I think the most desirable car is a personal choice. But based on the numbers, it looks like 1966.

Billy