I think it was the combination of the unexpectedness of it, the inevitability of it, the whole business of waking up in an unknown location in an unfamiliar city with people I knew only to a certain degree, the fact that I felt like I was just a queasy green colour, and the struggle to figure out what had happened and what would fix it all. My mouth was totally dry, and for whatever reason, London's delicious water did not seem to help it.
Most of my memorably bad hangovers are the ones where I'm only aware I've taken it too far when my full awareness suddenly returns to me from utter oblivion, and I just think "ohhhhh boy..." like the start of a rubbish Quantum Leap episode.
The other notables:
Toga party in a hostel in Rome, chatting up a nice girl until 5 in the morning, unaware that the bloke who was being so generous with the vodka was her boyfriend until he eventually told me to get the hint, but vaguely aware the two Irish lads were turning a bit nasty towards me. Woke up late afternoon with my mate extremely pissed off he couldn't leave me to go exploring in case I died or something.
Office Christmas party in a luxury Scottish country house hotel, chatting up a nice girl from another company, unaware that the bloke who seemed to be getting offended by my jokes about Germans, accountants and German accountants was - you've guessed it - the German accountant boyfriend of the nice girl. Thinking it must be about 11pm, I decided to move to whiskies, only to discover that they were shutting the bar because it was 3am. Back to someone's room for room service sandwiches and as much of my emergency hipflask whisky I could manage it, which was all of it. Woke up feeling terrible, and discovered my new nickname was "Black teeth Kenny" because I'd drunk so much red wine my teeth were black.
Lanzarote, a couple of years back. Final night. Nice meal to end a lovely holiday with my girlfriend. Got chatting to a couple on another table (not, I should add, chatting her up unaware of her boyfriend). The waiter started giving us free shots. Woke up back at the Villa, sprawled diagonally across the bed, girlfriend asleep in a huff on a pool lounger. She'd had to walk me up and down the promenade for an hour before I was sober enough a taxi would take me, and even then I was barely ambulatory. Not the most pleasant flight home.
My worst hangover, and I don't remember any other details such as where or when, I slowly became aware that I was alive, I was some sort of living... thing, and I didn't know what kind.
Then there was the time I was with my older brother and his friends, serious drinkers, and we were driving back from an aborted camping trip on the beach at Cape Cod national park, after we were rousted by park rangers who came up on us by boat, shone a big fucking light on us and yelled at us with a bullhorn to get the fuck out. As we were speeding the 6 or 8 hours turnpike drive home in a dilapidated converrtible with the top down, me riding shotgun, I vomited copiously over the side, which sprayed back on my brother, and his friends. This song came on the radio...