A classic car is a car that, in my opinion, meets one or more of the following criteria.
1. Was made in limited numbers.
2. Was hormoligated for a specific purpose.
3. Was a front runner or started a trend in something.
4. Was raced or has professional racing history.
5. Was an active prototype for a continued series.
6. Was constructed by hand or used specific techniques in their build.
7. Was owned by a specific figure. (Person) (Although this is open to abuse.)
This makes the VW MKI GTI a classic as it started the trend in hot hatchbacks.
This also make the VW Golf MKII G60 Rallye a classic as they were both made in limited numbers and were hormoligated for a purpose, and that was for competition.
MKII GTI's. whether they are 8V, 16V or G60, although desireable oldies, are not classic cars.
The Porsche 911SC, Carrera, 964 etc, are not a classic cars. But the 2.7 R.S is, as is the 959 and the 356.