If there is one soup that is an absolute must for me in the summer (in addition to Gazpacho of course), it is the cold, silky, incredibly reviving amalgam called Vichyssoise (vih-she-swaz). I have been known to make a gallon at a time and polish it off easily in a week. (Luckily, one can actually survive healthily on a diet of only potatoes supplemented with dairy.)
I have tested and retested this soup many times over the years, adjusting the key elements to create a soup with good body and memorable flavor. The onion element, for instance, began as a mere hint of flavor and then expanded, test after test, to a rich depth that fully complements the potato without overwhelming it completely.