For me, in the UK, it's London. I know it has absolutely amazing museums, theatre, culture, so much to see and do. I know it's an amazing place - for some people. I hate crowds too and don't like being around tonnes of people. It felt like everything was packed to the rafters - get on a bus it's completely full, the tube is rammed, every bar was rammed, going out to eat it's rammed, every seat is taken, etc. It is actually very overwhelming if you were brought up rurally and live in a very quiet place where there's about four people on the high street after 5:30pm. It got to the point where I can't manage the tube so had to get busses and sit by the window but kept wanting to get off them, one of the last times I was there I gave up and ended up getting taxis to places which cost a lot of money. I just felt like I couldn't breathe there. It also took longer to get there than to Spain.
Walk to bus stop - 10 min
Bus to city centre - 1 hour
Walk to train station - 15 mins
Train to Euston - 1 hour 24 (fast train)
Bus from Euston to friend's flat - 1 hour
Walk from bus stop to friend's flat - 5 min.
The problem was the train was cancelled and replaced with a bus that went to every stop. So that took 4 hours. And then I had to wait about 40 minutes for a bus on the way back. The timetable above is without waits!
I used to do this journey when I was healthy and I just treated it as a holiday, tried to relax and enjoy myself, would have food and drink in the train station to try to break it up a bit. But it is a monster journey.
So that is the journey but not the city itself, so you can't blame the city for that at all (although for me it's obviously associated with it as it's part of the holiday). But the issue was when I was there I just found everything difficult and busy. But, I will also say I had some amazing times in London, so maybe it's more a reflection on me, and when I started to find walking difficult/was in pain, it made things so much harder and my patience for the journey went. Also, if you can't walk without pain, London is tricky, as people are often on their feet the entire day.
Abroad, the worst I heard of but haven't been (family went) is Luxor in Egypt, because they couldn't leave the hotel grounds in the evening, to go on a walk or anything like that, and they said in the day they were hassled a huge amount. Also had a friend go to North Korea which I thought was bonkers!
I did not like Brooklyn, New York. The place I stayed had cockroaches.