To me a 'classic' is one that brings back memories of a certain bygone era, one that is no longer in production, one that is hard to find and most importantly, one that looks fantastic.
My classics would be original Boeings like 707,727, 737 and 747 probably not beyond 200 series...
DC8, DC-9, DC-10
Airbus is still a bit new but maybe the original A300 would count.
Also things like Tridents, Bac 1-11's Viscounts, HS748's, certain Soviet types from the 60's and 70's too...
Did I mention Concorde ?