Both were memorable!
Shortest cruise -- a "freebie" on NCL, pulled out of Long Beach, circled around overnight, pulled back into Long Beach. Amazing cruise. Some people simply did not go to bed! Just sitting in the bar and watching the drunken fools was ample entertainment, no show needed.
Longest cruise -- just got back from it, 18 nights from Sydney around Australia and the Great Barrier Reef, then up through Indonesia with Komodo, Bali and Java, ending in Singapore. Wonderful cruise, but at the end it was very nice to get home! Check out our trip report and pictures at Australia and Indonesia Cruise