Boring here. Never having flown anywhere but domestic (US) flights. We’re east-coasters - so a few flights to California on the order of 5-6 hours.
If I count the time I went to Hawaii, it’d add up to about 12 hours (plus layover time).
LA to Sydney for me, too. And I’ve done Auckland - Fiji - LA - London as a single itinerary, but with a two-night stopover in Fiji and a one-night one in LA, so that wasn’t so bad.
I really want to go back to Australia (if they ever let Americans in again), but the process of getting there is daunting.
13 hours flight time between Toronto and Beijing. Unfortunately we were diverted from Beijing to a nearby “domestic” airport due to weather. Because there were no customs and immigration at that airport we couldn’t deplane. So we just sat on the tarmac for 15 hours until a replacement crew was bused in from Beijing. Then we flew to Beijing, where there was a huge backup at gates and we spent 3 hours on the tarmac.
All told we were on the plane for 30 hours.
Later we found that the airport we had been diverted did indeed have customs and immigration facilities. Either the airline or the authorities didn’t want to deplane 400 passengers there for some reason.
Semi-off-tipic-ish, regarding connections, I flew on four consecutive calendar days in the same direction, all on the next flight available. Houston to Tokyo Tuesday, to Guam Wednesday, to Manila Thursday, to Cebu Friday. Also got to use a three-hyphen word.
Longest non-stop was on a Qantas A380 from Sydney to Dallas-Ft. Worth, about 16.5 hours from wheels up to touchdown. I’ve also had quite a few in the 12-15 hour range, Detroit-Bejing, Atlanta-Dubai, Chicago-Abu Dhabi, Atlanta-Johannesburg, Detroit-Seoul…got a few miles in.
If I may continue on this slight hijack…
In April, my kiddo kept getting additional “OK, now that you’re here, tomorrow fly to X” instructions from his company. As a result, he actually circumnavigated the globe, although inadvertently.
When I was a kiddo in the 60 s , the trans Atlantic prop planes , I seem to recall, From the States to the UK had to stop in Ireland . Also my mother had to wear a dress , girdle, nylons, gloves and a hat. My sister and I wore identical white blouses, skirts, anklets and mary janes.
Hong Kong to Newark, don’t remember the exact details but the current schedule for the flight show 15:45. It just seemed like forever. Economy Plus helped slightly…
Narrowly escaped what I’m certain would be the longest in subjective time. Middle (transatlantic) leg of a trip from Tucson to Tel Aviv - I think around 8 hours. Booked at the last minute, so not much choice of seats. Middle seat in economy, bulkhead. Al least I don’t have to worry about the person right in front of me reclining into my face.
The the fight attendant comes by with some sort of contraption and plugs it into the bulkhead right in front of my seat. “What’s that?” I ask. “A bassinet.” The woman in the aisle seat to my left was traveling alone with an infant. The flight attendant must have noted my look of abject terror - when boarding completed there was one empty aisle seat left and she moved me there.
Around 12 or 13 hours. From Adelaide, South Australia, to Hawaii.
Chicago-Osaka, 13 hours. We were in the back of the plane. I have to say, the back of the plane on an international flight is more comfortable than the back of the plane that we sat in Las Vegas-Atlanta last fall.
I’ve done San Francisco to Seoul half a dozen times once in first class (I was like 7 so I barely remember it besides falling asleep sideways in my seat).
Boston to Munich, Denver to Panama City and other reasonably long flights but like many other my long flight is LAX to Sydney.
I’ll sit anywhere for a short domestic hop, but for a long-haul international flight, I want to be near the front; it gets me off the plane quicker, and also gets me to/through customs quicker.
Detroit to Japan (either Tokyo or Nagoya) is 13+ hours. The first time I did it was about ten years ago, and it was far outside of my normal flying experience; by the end I was actually experiencing low level anxiety, as it was starting to seem like it would never end. Being awake for nearly 24 hours at that point didn’t help either; I just can’t sleep much on a plane. Subsequent flights to Japan no longer incur that sort of anxiety, as I know what to expect from the experience.
Wife flew back from Japan once and diverted to Cleveland because of a major winter storm; they didn’t deplane there, just sat on the tarmac until Detroit opened again, and then flew home. So she ended up spending something close to 17 hours on that plane, with several hours of it being spent in uncertainty about when it would end.
Chicago to Hawaii!! 
Los Angeles to Auckland, New Zealand. About 15 hours. In coach. The only thing that made it bearable was my seatmate, a biologist traveling to Antarctica to study seal breeding. Interesting conversation.
I did about 17h from LAX to Singapore in coach and it was … not fun. First off I hate using airplane bathrooms but obviously there’s no way to avoid it on a 17h flight. So I tried to strategically select a seat that gave me easy access to a bathroom and optimal comfort since I naturally struggle to fall asleep on planes. The seat itself wasn’t too bad but a new mother and her baby were seated about 4 seats away and she was pacing the floor with her crying infant in front of our row of seats for most of the flight 
If you don’t want to use the bathroom, you should at least get up out of your seat every hour or so to restore blood flow to your lower legs; sitting motionless for hours and hours puts you at risk for deep-vein thrombosis.
To deal with the baby (and all other troublesome ambient noises), I recommend wearing earplugs.
Atlanta -> Tokyo is over 14 hrs depending on the jet stream, and how many passengers actually showed up that day.
I start getting wiggy after the 10 hour point, no matter how comfortable or spacious it is. Can never sleep either.
The non-stop from Newark to Bombay is about 17 hours. The return flight was grueling as hell - not only did we have a 17-hour non-stop flight, we also had a 10-hour layover in Newark with an 18 month old and then another flight from Chicago to St. Louis. I remember being desperate to sleep but my son just couldn’t. He sat on my lap and my husband, who sat across the aisle from me, was so exhausted I couldn’t wake him up. I’d been up for 36 hours and put my head back and cried the whole way from Chicago to St. Louis because my son just couldn’t settle enough to let me sleep and my husband had sworn up and down he’d take him for just 45 minutes so I could sit without being touched or bounced or squirmed on, but he didn’t.
Now whenever we go to India, we stop somewhere in Europe just to break things up. The kids are older and so much easier to travel with, but being stuck in a plane that long even now gives me hives. I just can’t do it.
London to Dhaka, Bangladesh. It took 12 or 13 hours, but it was hideous. It was a trijet (DC-10, I think), packed, with a child screaming most of the way. Not one child…they worked in shifts. But you know what I mean.
Multiple tips between Seoul and Washington DC. Flight times varied because we re-fueled in flight.
Longest flight based on level of discomfort was Spain to the UK on a C-130 while hungover.