I doubt there is an official definition of "classic car." But here's mine: "A really cool car manufactured at least 50 years ago." Of course, "cool" is pretty hard to define, too!
Ones that immediately spring to MY mind....
1955 Mercedes 300 SL "Dual wing"
1960 Ferrari 250GT California
1938 Cord 812
1960 Jaguar E-Type
1965 Porsche 911
Any Duesenberg
1965 Shelby Cobra
.