Three pop to mind. One was an Alaska 737 flight in coach from YVR to LAX on Memorial Day, where I had the window, an old vet had the middle (and didn't want to switch), and a Canadian-Dutch woman had the aisle. We ended up talking the entire flight about the world and politics, and it was interesting since we were all from different backgrounds. The convo started when the vet picked up an application for an AS Visa and I told him to look online for a better bonus

Second was a Delta LAX-JFK flight in biz on a 767, my seatmate was just a bit older than me and worked for a large electronics/entertainment company. We got to talking about his work, which seemed interesting (I was 21 and unemployed at the time), then proceeded to drink the plane dry, as we were both massively delayed and the flight took about 6 hours. We had an FA who looked like a younger Deltalina stop by our seats plenty of times to quietly chat and fill up. She taught me the Biscoffs/Lime "recipe" that I still do to this day.

Third was another Delta LAX-JFK flight in biz on a 757, this one a redeye, and I was seated next to a drop dead gorgeous woman. She asked me if I had an iPhone charger, and since my phone was at 95% or so, I let her use it during the flight and that was the in. Talked over drinks about random stuff; combined with a very drunk Russian woman across the aisle (who had to be escorted back to Economy for the remainder of the flight ) and a near medical diversion from coach, and that was definitely one of my more interesting flights. I meant to sleep on that flight, ended up temporarily falling asleep in the shower at the JFK SC and almost missing my connect to DCA.