My most uncomfortable flight ever was to Israel – I booked it at 9 pm and was on the flight by 7 pm the following day. I got the last bookable seat on the plane.
I was 16 at the time. My seat was a middle seat between two large men. The man by the window was regular large (as in, slightly spilled over the seat but not morbidly obese), enough to encroach my space and make my uncomfortable. The other man was enormous enough that he should have paid for two seats. I was going to cry when about an hour into the flight, he took off his belt and unbuckled his pants. I guess he was really uncomfortable. I slept as much as possible.