Royal Cinque Ports.
That landscape, which allows nine greens to be bunkerless without you really noticing, because there isn't a single one that doesn't have challenge and interest in spades.
The lack of length without it being a walkover - I still challenge anyone to shoot their handicap from the 6300-yard par 71 tees.
The width of most of the corridors.
The amount of short grass around the greens.
It established a prism through which I have viewed every course I have seen since.
EDIT - John: Great minds. You count only eight bunkerless greens? I have the list as 1, 3, 4, 6, 10, 15, 16, 17, 18.