I would have to say our 64 day "World Cruise Liner" on the Sea Princess in 2015. It was booked as one cruise, but it was three segments of the World Cruise. We flew from San Francisco to Sydney, Australia. We spent four days in Sydney, pre-cruise. Then around southern Australia up through the Suez Canal, Eastern and Western Mediterranean, British Isles, Iceland, trans-Atlantic, ending in New York. Then flew back to San Francisco.
We love repositioning cruises, adding an itinerary or two B2B2B before hand on both Atlantic and Pacific crossings. This was that on steroids!