7th Jan DME - LHR (can't remember which year) on a shorthaul 767. (Before the route went long-haul) This was Christmas day for the Russian passengers - I was in 1D and a stranger in 1F. By the time we landed in LHR, I am sure the whole of the front cabin knew each other. The crew were brilliant throughout, with loads of top-ups and even trying to sing along with some of the songs being sung in Russian.
I think that because there were no lie-flat seats on the route in those days, it was more convivial as all of the passengers could see each other. I have loved many flights with BA, including obviously my first First, BA1, my first 747 upper deck, but they have all been eclipsed by a flight with loads of strangers on a slightly tired 767 with a tiny IFE screen