Good question.
A classic isnt something I consider to be a favourite song, but one that will be robust and stand the test of time. In that vein the last 'classic' would be something like (recent order first) Boom Like That, Postcards from Paraguay, Our Shangri La, Hill Farmers Blues, All That Matters, Why Aye Man, What It is, Saling to Philadelphia. Speedway at Nazareth and nearly all of the tracks from Golden Heart :-) - plus a special mention for Kill To Get Crimson - a favourite album of mine, no skippers, but no classics - just every song is very listenable.