#5) 356 Speedster: It spite of the fact that it didn't look like anything before or since and not that many were made, somehow it became an icon. That sounds like a classic.
#4) 911SC: Where the earlier 911's were sort of specialist cars, the SC really made the 911 accessable to the masses and became synonymous with "Porsche" to the world over. (Gosh, I sound like a member of the SCWDP, don't I
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#3) 917K: Show a picture of it to just about anyone the world over, it they'll most likely say "Le Mans". It came to define Porsche's steam roller reputation in Racing.
#2) 935-78: The ultimate 911. In befor it there had come other great 911's (Such as the Carrera 2.7 and 930), but the 935-78 came to be the high point of Porsche's creative interpretation of the FIA's rules. It was just too wild. Only 2 factory cars were made, but it was widely copied. Since then racing Porsche's have just been Porsche's built to the rules, the -78 was an excercise in going to the limit.
#1) 1964 911: You can't build something in one form or another for almost 40 years and not call it a classic.