but the flight that 'seemed' the longest that I've taken was to Japan. I believe a little over 11 hours.
Another flight I remember taking back in 1969 took 11.5 hours... direct flight from London to LA.
Most of my other international flights had at least one stop.
My most unforgettable flight. When I was in the Army I flew cross country several times between Newark, NJ and either SF or LA. I usually flew what I think they called the 'red eye'. On one of these flights I flew in a 747... this was back when the upstairs of the 747 was a bar. There were exactly 7 passengers on board the plane. All but one of us spent the entire flight in the bar.
If I'm not mistaken there were more crew members onboard that flight than passengers. I remember asking a stewardess how they could afford to do that and she told me that it rarely happened but by law they couldn't just cancel the flight due to few passengers.... and in the end it didn't really matter because they made up for it in various other ways.
One of the pilots joined us in the bar for a short time, and no, he didn't drink. At least he didn't at the bar.