In February 2018 I was on UA37, LAX-SIN. In distance it would be a ways down the list. However, it is not a polar flight, so it flies pretty much west, which I guess subjects it to headwinds. The flight time would be a bit closer to the top. I used a stopwatch to time the length of the flight from leaving the tarmac in LAX to touching down in SIN. It was 17h 34m, off the ground.
United flew it with a B787-9. I was in regular economy, and it was not a problem at all. I am tall, but I had an aisle seat so I could stretch a bit.